Abstract: There is deformation problem in large area diamond thin film substrate prepared by hot filament CVD method, the problem of silicon wafer with diameter of 76 mm and thickness of 0.4 mm was resolved successfully by improving the equipment of hot filament CVD and the deposition process. The results show that the angularity of the diamond thin film substrate after improvement was 0.296%, reduced by 88.9% comparing with before improvement. The quality and grains size of the diamond thin film after improvement was uniform, the unevenness of the thin film thickness was only 1.53%, and excellent uniformity of large area was obtained.