Measuring method of phase delay of wave plate is just the average for the laser beam(about 0.5 mm in diameter).In this paper,the detection of spatial phase delay for large diameter wave plates,the detection method based on Fizeau interferometer is proposed,and the theoretical mathematical model between the space phase delay error of the wave plate and the interference pattern is established.The main factors affecting the phase delay are analyzed theoretically:the spectral width of the light source,the spatial refractive index distribution of quartz crystal,the face shape error of the wave plate.The numerical calculation is carried out by MATLAB programming,the results showed that:the spectral width of the light source should be less than 0.2 nm,the spatial refractive index distribution error of quartz crystal should be less than 0.005,and the surface error should be less than 200 nm,if the total error of phase delay of the wave plate less than 1°that of general wave plate test error;The Fizeau interferometer was built in the lab,and the test of quartz wave plates with a diameter of 25.4 mm was carried out.The test results are good,and the measurement precision is 0.05 °.