A Quasi-optical analysis of FY-3 satellite radiometer antenna system is described. Equivalent electrical and magnetic current sheet were used for the analysis of wire grid cross polarization which could be down to -42 dB from the main polarization. A Gaussian coupling method was then introduced to analyze the offset parabolic mirror which was fed by a corrugated horn at 90º. As long as the reflector's f/D is large, the coupling matrix which is used to model the offset effect of the reflector can be derived in a simple way. The simulations and measurements demonstrated that quasi-optical analysis can be applied to offset reflectors well with its efficiency and accuracy.