Exact enumeration method is used to calculate the conformational entropy of polysilane chain, submitted to short-range interaction and long-range interaction, with an off-lattice RIS model. The conformational entropy is in proportion to chain length at any temperature (from 50 K to 400 K). It varies with temperature, can be expressed as the function of temperature. And we find the second-order transition temperature (T2) of polysilane is about 65 K.