The purpose of this paper is to solve the positioning problem of a double body working ship. Dynamic characteristics of mooring system are analyzed with the effect of wind, wave and current. According to the dynamic equations of the system in the horizontal plane, longitudinal force coefficient, lateral force coefficient and yawing moment coefficient are obtained by towing test. The wind lord, wave load, current load and chain force can be calculated by empirical formulas. The motions of a mooring system are simulated in MATLAB. The results show that yawing motion would be occurred for the mooring system with single anchor at one point. The motions of the system have similar forms under the same amplitude of external loads. The results are agreed well with the observation data from documents about this relation. It is also can be point out that single point mooring system has poor stability and can not control the motion accurately. Thus dynamic positioning/mooring system should be developed for anchored working ships. This research in this paper provides the theoretical support and the analysis foundation for the further studies.