The purpose of this paper is to improve the automatic collision avoidance procedure of two ships stipulated in open sea areas without the disturbances of wind, wave and current, which is achieved by altering course based on ship dynamic domain. As the rapid increase in the study of ship collision risk index (CRI) and automatic collision avoidance decision-making for ships, the decision-making process is fully recognized. In this paper, the model of ship dynamic domain are adopted and a series of simulation studies are made. The safe distance of approach (SDA) is determined by ship dynamic domain. Then ship automatic collision avoidance procedure is established to controlling course alteration for collision avoidance. Experimental results show that ship dynamic domain is suitable to collision avoidance. The key factor that leads to the in validation is the CRI and course alteration. Based on the seagoing experience and simulation, the decision-making for collision avoidance in three encounter situations is accurate enough for this application. The results provide a reference that compute and display the ship parameters fast and accurately to decision-making process of ship collision avoidance.