This paper uses SIR model to back-analyze the parameters of SARS transmission based on the data released by the health authorities of Beijing and Hong Kong, we get the important parameters such as the peak period, the hospitalized cases and the removed parameter. It can be seen that these parameters of the model allow for better understanding of the SARS transmission because the result fits the actual data approximately. As a result, SIR model could be used to fit data, predict trend and simulate process of SARS transmission.