In the international underwater robot competition, a circle algorithm is introduced in view of the uncertain factors that appear during the competition process of the 2D simulation of the survival challenge of the robotic fish. Through the analysis of the rules of the 2D survival challenge, this paper studies the tactics and algorithms, mainly expounds the defensive tactics of the attacking and dodging fish, among which, two basic strategies for escape and perforation of the fish are avoided. In addition, this paper also introduces the circle algorithm of special position treatment and avoidance capture. The results show that the method is effective and feasible, especially when defending fish happens to withstand the attacking fish.