This paper proposes a mobile agent based composite service execution mechanism to achieve the process oriented composite service represented by BPEL4WS.This paper also gives a detailed process of execution of composite service.Mobile agent based composite service execution is a P2P(peer-to-peer) execution model which can avoid the performance bottleneck of center peer and effectively reduce the network load.Due to the cost of agent migration,this paper investigates and designs a mobile agent resident strategy to avoid unnecessary migration,which raises execution efficiency greatly and reduces the resource consumption.