SBDART radiative model and libRadtran software package are introduced. Cirrus reflectance changes with ice crystal properties are analyzed. These properties include ice effective radius, ice content and ice shape, etc. Simulations show that cirrus reflectance is different when it is made of different ice crystals. In all, the results are that cirrus reflectance increases with increases ice content and decreases ice effective radius for the cloud which consists of any kind of ice crystals.