A new method based on extended Nijboer-Zernike (ENZ) theory is proposed for testing the wavefront diffracted by a pinhole in the point diffraction interferometer (PDI). The principle of PDI is described and the phase retrieval algorithm based on ENZ is put forward. The way in expansion of the pupil function with Zernike polynomial is analysed. In the simulation experiment, deviation in the image part of the Zernike coefficients obtained from the retrieval result is less than 3×10-5, when singal-to-noise (SNR) is 55 dB with a 10-bit analog-to-digital conversion. The effect of noise and the analog-digital conversion is only involved, the result in this simulation experiment just proves that testing the wavefront diffracted by a pinhole with this method is possible.