To solve the problem that a single optimal target anycast mode tends to cause new load imbalance, a Measuring & Controlling Network Platform(MCNP) based on the Session Initiation Protocol(SIP) was established and its anycast mechanism was analyzed. By integrating network-layer anycast and application-layer anycast, an load-balancing method based on an optimal decision set was introduced.The anycast objective is a set,which includes many servers to avoid server resources to be seized confusedly. To obtain an optimal decision set, a mathematical model of optimal decision set anycast was established,and the evidence theory was used to anycast the weight. An experimental platform was built to simulate and verify this new method in OPNET simulation environment. The results show that the imbalance rate of server resource occupancy by using the optimal decision setanycast mode has reduced by 31% as compared with that by using a single optimal target anycast mode, which brings an equal distribution of load to different servers and reduces the extra resources for tackling multi-processing in a single individual server.