The reflectance of cirrus clouds is an important parameter in weather, climate and earth energy balance studies. The fast algorithm for calculating the reflectance of cirrus clouds plays an important role in the retrieval characteristics of cirrusclouds. Based on the variations of the reflectance of cirrus clouds with the relevant factors, such as cirrus optical thickness, effective particle size, solar zenith angle, observing zenith angle, the relative azimuth angle et al, a prepared Look-Up-Table(LUT) was developed by using DISORT to compute the reflectance of cirrus clouds. A fast algorithm for calculating the cirrus reflectance was established with utilization of the LUT. The cirrus reflectivity was calculated from the parameters of cirrus clouds, including cirrus optical thickness, effective particle size, solar zenith angle, observing zenith angle, the relative azimuth angle, which were read from the cloud products of MODIS. A comparison between the derived reflectivity with the proposed fast algorithm and the measured one was performed. The correlation coefficient is 0.94 and an average deviation is less than 18.5%. The result indicates that the fast algorithm for calculating cirrus reflectance is reasonable and feasible.