With the development of technology and the increasing complexity of battlefield environment, the demand of intercepting maneuvering targets is increasing. However, the traditional guidance law has the limitations of guidance accuracy and terminal overload mutation when intercepting maneuvering targets. Therefore, an optimal guidance law based on fractional calculus theory is proposed in this paper. Firstly, the definition, properties and numerical implementation of fractional calculus are introduced. Then, the fractional order guidance law is derived by fractional order modeling and optimal control theory. Finally, the simulation results show that the fractional optimal guidance law designed in this paper could maintain the good tracking performance of the proportional guidance method, the interception time could be shortened by 2s, and the terminal overload value approaches to 0. It solves the problem of terminal overload mutation caused by the divergence of line of sight angular velocity in the traditional proportional guidance method.