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In order to measure high accuracy or mid spatial frequency optical surfaces, this study investigated imaging system in Fizeau interferometer. Interferometer is a transferring phase information system. General transfer function can't evaluate the ability of imaging system in interferometer that transfers the phase information of the optical surface. So this paper deduced instrument transfer function (ITF) and used ITF to evaluate the phase information transfer ability of interferometer. We obtained some results about ITF of imaging system by numerical simulation when there are different aberrations in imaging system or measuring optical surfaces were different. Our results indicate that ITF can evaluate the ability of transferring phase information in interferometer and the distortion of imaging system evidently affects the accuracy of interferometric measurements.