The surface accuracy of lithography projection objectives is on nanometer scale,so the accuracy of the testing interferometer is also needed on nanometer to sub-nanometer scale. A method to test the optical surfaces of lithography projection objectives using Fizeau interferometer is proposed. Through theory analysis and computer simulation,the system errors caused by phase shifting,reference surface,and detector nonlinear are investigated. Other factors affecting measuring accuracy including light source instability and environmental control are also analyzed. The relationship between measurement errors and interferometer structure parameters is presented. Some methods and measures for improving accuracy and decreasing measurement errors are put forward. The results of the computation show that the accuracy of the interferometer is mainly limited by the accuracy of the reference surface and the stability of the environments. The results also have an important reference value for the design and manufacture of Fizeau interferometers.