There are noticeable errors at the border of the pattern when the traditional Fourier transform method (FTM) is used to evaluate the carrier fringe pattern. A new extrapolation of fringe prolongation method based on the shift theorem of Fourier transform is proposed. Numerical simulations are performed to evaluate the performance of the method which reduces the errors at the borders of the pattern. An accuracy of the phase evaluation of approximately 3.3 mrad is obtained. In order to verify the methode, a radial shearing interferometer system has been set up. The surface of a plano-convex lens has been measured. Wavefront traversing the sample is reconstructed by means of the new extrapolation method of fringe prolongation. The results show that errors are reduced in the processing of shearing wavefront testing.