Spatial Heterodyne Spectroscopy (SHS) is a novel technology which shares the advantages of grating diffraction and spatially modulated interferometry. However, the flat-field of the system deviates its theoretical values due to the errors from interferometric gluing and grating etching. This paper explores the calibration principle of the flat-field of the system for the SHS and derives its basic formula by the interferomatric fringe frequency formula. Then, it discusses the principles of zero frequency calibration and designs a new zero frequency calibration device based on an integrating sphere introduced by a tunable laser for eliminating speckles. A calibration experiment is performed on a SHS prototype for atmosphere CO2 fine absorption spectra in Anhui Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences. The calibration result suggests that the instrument′s zero frequency drifts 0.05 nm toward shortwave as compared to the theoretic value, which meets the design requirements.