Abstract: The beam calibration results are affected by the impedance of the calibration device. If impedance matching is not maintained, reflected signals would be generated, which would lead to the superposition of reflected signals and excitation signals, therefore, the signals that stimulated BPM are not similar to the real beam signals. In addition, the unsealed outer conductor of some beam calibration devices may lead to leakage of excitation signals or detection of other electromagnetic signal noises in the laboratory, thus increasing the calibration and detection errors. In order to calibrate and measure the transverse bias of the beams in the Rhodotron electron accelerator more accurately, an external moving type beam calibration device based on quasiimpedance matching transmission was proposed, and its outer conductor was completely enclosed. The connection of the main components for the outer conductor is as follows: excitation signal input portfirst fixed straight line sectionfirst bellows sectionfirst impedance matching section-BPM-second impedance matching sectionsecond bellows section-second fixed straight line section-excitation signal output port. In order to study the calibration device, CST MicroWave Studio was used to establish the calibration device, and the beam bias was simulated by fixing the inner conductor rod of the calibration device and moving the beam detector (outer conductor) outside the calibration device, so it is called external moving type calibration device. The ratio between the inner diameter of the outer conductor and the outer diameter of the inner conductor is kept at 2.303, so that the impedance matching of the electromagnetic wave in the transmission of the calibration device is maintained at 50 Ω. The influence of outer diameter of inner conductor on the spectral response of detector was studied. The results show that changing the size of the inner rod does not change the resonant peak position of the beam detector. The inner conductor rod is fixed and the outer conductor is moved for the first time to maintain the quasiimpedance matching state of the calibration device. In the impedance matching section, the aperture of the two ends of the pipe can be changed to adapt to BPMs of different apertures. To calibrate BPMs of other aperture, only the impedance matching section and the inner conductor rod need to be redesigned, thus increasing the utilization rate of other equipments of the calibration device. The effect of external moving type calibration device on beam calibration was compared with that of fixed outer conductor and moving inner conductor calibration device. The results show that the detector spectral response curves of the two devices coincide highly regardless of whether the beam is offset or not, and the outer conductor moving calibration device has a better impedance matching. Finally, the outer conductor moving calibration device was applied to the actual beam calibration, which proves that the calibration device could be used for the calibration of Rhodotron electron beam.