LAMOST is a special reflecting Schmidt telescope. Both the refleting Schmidt plate $M_A$ and the spherical primary mitrror $M_B$ are segmened mirrors. These two cofocus but not co-phase. The diffraction of the optical system is decided by the shape overlapping of $M_A$ and $M_B$. This paper describes the diffraction caculating results with different declination and different field angle. The diffraction influence to the image quality is acceptable in the error buget of optical system. It also proves that the size seletion of the sub-mirror is reasonable.