Aiming at the problem of undersampling in the aspheric testing, a method for phase recovery from a single undersampled wrapped phase map is presented. Firstly, two wrapped phase differences are gained by subtracting the original undersampled wrapped phase with its two sheared copies along two orthogonal directions while the sheared value is 1 pixel. Secondly, the unwrapping is performed for the two wrapped phase differences. Finally, the searched phase is obtained by using the Fourier transform and least square algorithm. The simulation results show that the method can retrieve the wave-front with high accuracy. Using the method, the aspheric surface can be tested by current interferometers.