Mach-Zehnder point diffraction interferometer is used to reconstruct the complex amplitude by obtaining the information of phase and intensity of laser mode from a interferogram. A method to reconstruct the complex amplitude by using point-diffraction interferometry based on fast Fourier transform is analyzed theoretically. The phase and intensity distributions are deduced when laser mode propagates in the free space. The simulation of phase and intensity distributions with the experimental results are compared. The results show that complex amplitude can be restored by point-diffraction interferometry.