PIM-SM (protocol-independent multicast-dense mode) is currently the preferred intra-domain multicast routing protocol. One problem that impedes its widely use is its high overhead of control messages. In order to improve and optimize the PIM-SM, its performance model should be established and the nicety performance analysis should be made above all. In this paper, the Stochastic Petri Net (SPN) model of the whole PIM-SM protocol is established, and the analysis and the experiments are made on the router processing load caused and the network bandwidth consumed by each type of the protocol messages, based on the model and router realization. It is discovered that register message and Join/Prune message cause most router processing load, while Join/Prune message and Bootstrap message consume most network bandwidth. According to the conclusion of performance analysis, an improvement is made on PIM-SM, which is achieving better performance compared with the former protocol.