In view of the characteristics of Ka-band mobile satellite channel, a model was built up with taking full consideration of the decline influenced by weather and surrounding environment in this paper. Then, the satellite channel with Bipolarity Phase Shift Keying (BPSK) modulation was simulated, and the probability model of baseband channel bit error distribution was established. Meanwhile, the probability of error occurrences was fitted by using the least-squares method. According to the results, the occurrence number of channel errors obeys Poisson distribution. Moreover, during the digital baseband simulation of Ka-band mobile satellite channels, it is found that the system error with BPSK modulation consists of the burst error and the random error that obeys the Poisson distribution.