In this paper, a generalized method to efficiently represent the incident-angle-dependent mask transmittance function (MTF) of a thick mask is proposed. This method expands the MTF into a series expansion, which consists of a set of predetermined basis functions weighted by a set of predetermined expansion coefficients. The predetermined basis functions are independent of the incident angles and thus may be computed offline and stored, while the expansion coefficients depend only on the incident angles and can be rapidly calculated online. Near-field and optical image simulations of thick masks have demonstrated the excellent accuracy and superior speed performance.