Based on the difference between nomal grating and immersion grating,the immersion grating equation was deduced.Immersion medium absorption was studied as the reason of the single slit diffraction and the change of the multiple beam interference intensity distribution.The correction factors of single slit diffraction and multiple beam interference were concluded,and the intensity distribution formula considering the medium absorption was deduced.The formula is more general and the single slit diffraction intensity distribution formula,the multi-slit diffraction intensity distribution formula and the parallel-plate multiple beam interference intensity distribution formula are special cases of the correction formula.