In order to improve the imaging quality of modified super-widen-angle Sagnac imaging interferometer (SASI),the modulation transfer function (MTF) fMTF is investigated. After calculating the five boundary conditions of incident ray of the modified SASI,its pupil function is given. Then,the expression of MTF of the modified SASI is deduced according to Fourier transform. Using the relevant parameters of optimized SASI,the curve of MTF is drawn and the result is fMTF=0.62 at Nyquist frequency. But the fMTF of WINDII (Michelson imaging interferometer) which is used to measure the upper atmosphere wind field is 0.35. It shows that the SASI has a better imaging quality than WINDII.