This paper focuses on the complementary sliding mode control(CSMC) of fractional-order Buck converters. Firstly, to establish a more accurate model for describing the characteristics of the Buck converter, a mathematical model based on the Riemann-Liouville definition is proposed, which is more precise in describing the characteristics of the Buck converter compared to the Caputo definition, considering the non-integer order characteristics of electronic components. Then, to deal with parameter uncertainties and external disturbances, which are lumped as matched and mismatched disturbances, two fractional-order disturbance observer(FDOB) are designed to track them and their fractional-order derivatives. Subsequently, a novel fractional-order CSMC surface is developed to improve the robustness and steady-state error of the sliding mode phase by taking advantage of the high accuracy of CSMC and the memory property of fractional calculus. A new reaching law is also introduced to increase the convergence rate while maintaining the robustness of the sliding mode. Finally, the stability of the sliding mode controller is demonstrated based on the Mittag-Leffler stability. The simulation results demonstrate the superiority of the FDOB. Compared with the traditional sliding mode strategy, the proposed controller achieves better dynamic performance and lower steady-state error.