Diffractive micro lens can be used as a light spitting element in imaging spetrometer and micro spectrometer for its function of image and dispersion and its character of small volume,easy to integrate and array-with.The formula group for the design of multi-phase micro Fresnel lens according to scalar diffraction theory was given and the transmission theory of spherical wave were used to deduce the relation of focal distance of the micro lens with spectrum resolution.According to the relation spectrum resolution is inverse proportional to focal distance given a certain incident wavelength and phase number,and the theoretic formula is consistent with reported experimental result.