Abstract: Neutron detectors with excellent resolution, responsiveness, and radiation tolerance are desperately needed in the field of neutron monitoring. Diamond material has several advantages such as a wide bandgap, strong radiation tolerance, and high carrier mobility, making it suitable for neutron measurement applications. In this work, a single-crystal diamond neutron detector equipping with ⁶LiF thermal neutron converter was designed and fabricated to improve thermal neutron efficiency by detecting secondary charged particles produced by the ⁶Li(n,α)³H reaction. To improve the thermal neutron detection efficiency of diamond detector, Monte Carlo simulations were conducted based on polyethylene moderator, ⁶LiF thermal neutron converter and a single-crystal diamond detector for the ²⁵²Cf neutron source. Optimal thicknesses for both the moderator and the thermal neutron converter were determined through these calculations. The required thickness of polyethylene moderator for detecting thermal neutrons was determined to be 5 to 6 cm. Subsequently, a single-crystalline diamond neutron detector with a 1 μm-thick layer of ⁶LiF was fabricated based on these results. The single-crystal diamond neutron detector structure was designed as a metal-semiconductor-metal structure. In the central 3.5 mm×3.5 mm area of both the upper and lower surfaces of the single-crystal diamond material, Ti/Au electrodes were deposited by electron beam evaporation with thicknesses of 30/400 nm respectively. The material was fixed onto a PCB board and corresponding shell design was made. By magnetron sputtering for 24 h, a 1 μm-thickness ⁶LiF conversion layer was deposited onto standard copper nut. The coated nut was then combined with the detector to fabricate a thermal neutron detector. The energy spectral response of the detector to both the ²⁵²Cf neutron source and the D-T neutron source at 14 MeV was measured. The research results indicate that the fabricated detector, compared to the detector without the ⁶LiF converter, can achieve a maximum increase in the thermal neutron count rate from slowed-down ²⁵²Cf of 13.2 s⁻¹ and an energy resolution of 7.7%, which is superior to that of the Cividec B6-C diamond detector. The Cividec B6-C diamond detector has an energy resolution of 8.9% for the tritium characteristic peak under same test conditions. The fabricated detector demonstrates an energy resolution of 1.95% for detecting 14 MeV neutrons from the D-T neutron generator. These research results are expected to be applied in the calibration of existing ²⁵²Cf neutron sources and the measurement of 14 MeV neutron spectra. Additionally, they provide essential theoretical and data support for the fabrication of diamond neutron detectors and neutron beam monitoring.