This paper established the mathematical model of fractional-order BUCK converter firstly, and obtained that the model has chaotic characteristics under some certain conditions. Due to fractional calculus has widely development recently, fractional-order stability theory is applied to design the fractional-order PID controller in BUCK converter, and the order of traditional PID controller promoted to fractional-order field. In PID parameters tuning, the ITSE optimal algorithm is applied to design the fractional-order PID controller. At last, the controller is applied to the BUCK converter, simulation results show that the controller can improve the control performance and the stronger robustness.