The difference between the measurement angle and the use angle will produce measuring errors when the wavefront errors of the large reflective optics in a high power driving device are measured. So this paper proposes a method to convert the reflection wavefront in a measuring angle into the reflection wavefront in a use angle. Firstly, the cosine conversion method used in conversing the wavefront in an oblique incidence angle into the wavefront in the use angle was analyzed, and the calculation relationship of the wavefront errors between actual measurement angle and actual use angle was obtained. Then, the mid-frequency PSD1 (Power Spectral Density) error caused by the third interpolation algorithm itself was calculated, and it points out that the selected incident angle should be close to that the actual use angle as much as possible when the measuring condition meets the effective diameter measurement. Finally, the error analysis and experimental verification were carried out based on a 410 mm ×410 mm fused quartz reflector.Using this method, the 0° reflection wavefront was convered into the 45° reflection wavefront, and the measuring result after conversion was compared with that of the direction measuring result at 45°. The compared results show that the differences of PV values, RMS values and the PSD1 values are 0.01 λ, 0.003 λ and 0.08 nm, respectively. These results indicate that the method not only can calculate exactly the low frequency errors of reflection wavefront, but also can obtain the PSD1 mid-frequency error.