In order to measure the surface shape of large aperture plane mirrors by using smaller caliber laser interferometer, oblique incidence method is developed. Thus, the spot illuminated on the tested mirror is magnified, and the size range that the interferometer can measure is much bigger. The plane mirror surface shape expression based on oblique incidence is gained, and the errors that may be introduced are concerned. And a plane mirror with size of 124 mm×42 mm is measured using vertical incidence and oblique incidence with different incidence angles. The surface height root mean square (RMS) and peak-to-valley (PV) results of the test mirror are 16.3 nm and 67.8 nm based on vertical incidence and 16.8 nm and 68.7 nm based on oblique incidence. Compared with vertical incidence measurement results, the relative error value of RMS is 3%, and the relative error value of PV is 0.9%.The result can satisfy the requirement of the third generation synchrotron radiation beam lines.