A multi-agent decision support system for ship collision avoidance(SCA-MADSS) is presented in this paper.Its organization structure is designed and a formalized description of the structure is given.The concept of ship collision avoidance union and its construction principle are also put forward.The four kinds of intelligent agent model,i.e.Control_-Agent,Union_-Agent,Ship_-Agent and VTS_-Agent,are established separately,the BDI structure model of the Ship_-Agent is discussed specially.