Large sky area multi-object spectroscopic telescope(LAMOST) is a reflecting schmidt telescope, the unique
telescope structure make it compatible with both large sky area and large aperture. While the vignetting effect
of the telescope is not the same everywhere, the flat field is quite different from the common flat field which use a
lamp projecting to a fixed white screen. We describe the Schmidt reflector(MA) cover solution of the LAMOST
flat field, simulation shows the variation across the field of view is less than 2 percent. The material of flat field
screen is carefully chosen to have both high reflectivity and diffusion. We also describe the material test for the
LAMOST flat field screen. Flat field lamp should have high color temperature to raise the sinal in the shorter
wavelength. The possible solution for the flat field lamp is described.