Fizeau interferometer with the advantage of common optical path which the reference light and test light are passed becomes main structure of interferometer. The structure of interferometer and its influencing factors in theory have been analyzed by lots of literature. But how to simulate Fizeau interferometer is the problem we faced. This article first introduces the basic principle of Fizeau interferometer and phase unwrapping, and the phase unwrapping procedures based on Matlab were written. Then Zemax optical design software was used to model Fizeau interferometer and four-step phase shifting was used to simulate surface testing. By the phase unwrapping procedures, the true surface to be tested were got. The simulation of testing result is in line with the result given by real Fizeau interferometer. Simulation result shows that the model can simulate the actual optical interference.