Softswitch is the most complicated system in next generation networks. An ideal softswitch system can produce the best efficiency/cost ratio for telecom service provider of different scale and large enterprise. It is also easy to be extended and upgraded. Multi-agent based softswitch (MABS) is given to reach the target. The architecture of MABS is presented. The cooperation of agents in MABS is described. The protocol of communication among agents over Ethernet is designed as well. An implementation of MABS prototype is used to verify the design of this paper. The results of experiment verified the main functions of MABS and it shows good performance.