In order to break the bottleneck of conventional autonomous underwater vehicles (AUVs), biologically-inspired AUVs (BAUVs) have drawn great attention. This paper explores the turning control issue of a tuna-like BAUV with the purpose of seeking a minimum turning radius. Specifically, for better understanding of the turning motion, a dynamical model is established and the force acting on the tail fin is taken as the main source. Because of the complex nonlinear relationship and physical restrictions, it is hard to optimize the turning radius directly. To tackle with this problem, the input signals of joints are generated by sine function so as to satisfy physical constraints, and simulations under different parameter combinations are conducted. Furthermore, a fixed tail joint method and a half-cycle flapping method are chosen and compared in an indoor pool. Finally, the experimental results verify the effectiveness of proposed methods, and a minimum turning radius of the tuna-like BAUV is achieved as small as 0.37 body lengths.