Abstract: Magnetic confinement fusion is one of the most promising technical routes for developing future clean energy. To realize fusion ignition, it is essential to accurately measure the electron density of plasma, a key physical parameter in fusion reactions. The far-infrared (FIR) laser interferometer serves as a critical diagnostic tool for electron density measurement. A multi-channel Michelson-type FIR laser interferometer was developed for the HL-3 Tokamak magnetic confinement fusion device, with a time resolution better than 1 μs and a density resolution up to 5.0×10¹⁶ m⁻². The device was also equipped with a 3.7 GHz lower hybrid wave (LHW) system for plasma current drive and auxiliary heating. However, experiments revealed that the 3.7 GHz LHW introduced severe electromagnetic interference into the FIR diagnostic system. Specifically, the measured electron density signal exhibited abnormal jumps, and the original reference and detection signals presented severe waveform distortion upon the injection of LHW. Such interference distorted the measurement data, disabled real-time electron density feedback and seriously undermined the normal operation of the FIR system. Combined experimental observations, spectral measurements and a series of elimination experiments confirmed that LHW radiation leaked outward through the diagnostic window of the vacuum vessel, and the Schottky diode detectors inside the main optical box of the FIR system were the main affected components. These detectors have a central response frequency of 694 GHz and a working bandwidth ranging from 500 GHz to 750 GHz. To address this issue, the finite-difference time-domain (FDTD) method was applied to construct a two-dimensional transverse electric (TE) wave simulation model for LHW. This model complied with Maxwell’s curl equations and the standard discretization rules of Yee grid. The model referred to the real dimensions of metal pipelines, diagnostic windows and main optical box of the FIR system. Proper simplification was carried out while all critical boundaries for electromagnetic wave propagation were retained. The computational domain was divided into a 1 100×1 000 Yee grid. The spatial step equaled 1/20 of the LHW wavelength, and a total of 10 000 iteration steps were performed to obtain steady-state electromagnetic field distributions. The perfectly matched layer (PML) was adopted as the absorbing boundary condition, and the wave source was simplified and placed at the diagnostic window for numerical calculation. Two typical operating conditions including vertical and horizontal polarizations of incident electromagnetic waves were fully considered to conform to the actual complex radiation characteristics of LHW leaking from the vacuum vessel. The established model was utilized to investigate the propagation behaviors of LHW and analyze the internal electromagnetic field distribution within the FIR main optical box under the two polarization states. A metallic shielding baffle with laser apertures was accordingly designed to suppress electromagnetic interference caused by LHW while ensuring the normal transmission of laser beams. Simulation results demonstrate that the electric field intensity of LHW in the detector region of the FIR laser interferometer is reduced by 10-20 dB after applying the metallic shielding baffle. Specifically, horizontally polarized incident waves achieve an attenuation close to 20 dB, while vertically polarized incident waves reach an attenuation of around 10 dB. Based on the simulation results, a non-magnetic aluminum alloy shielding baffle with optical apertures was fabricated for the FIR laser interferometer on HL-3. This material delivers superior shielding performance through skin effect and electromagnetic interface reflection, and its non-magnetic property makes it well suited to the strong magnetic environment of the Tokamak. Experimental results are obtained through comparative tests with and without shielding. This shielding scheme lowers the interference level of FIR laser interferometer raw signals to approximately one-fifth during LHW injection. The shielding structure sustains steady interference suppression performance, even when LHW operates at a higher power than that in the unshielded case. The abnormal jumps on the electron density are eliminated, and the measured electron density curves accurately track plasma parameter changes and return to the normal baseline after plasma discharge terminates. The diagnostic system recovers stable operation. The shielding method can provide a valuable reference for the anti-interference research of diagnostic systems against lower hybrid waves on Tokamak fusion devices.