In order to ensure the stable operation of Stacked Multicell Converters (SMC), active control method is usually used to balance the voltage of flying capacitors. The active control method needs more voltage sensors to obtain the voltage value of the flying capacitor, which will lead to high cost and poor robustness of the system. An observer is a typical solution that can replace a voltage sensor and a sampling circuit to achieve the same effect. In this paper, a sliding mode observer method based on switched linear model is proposed to estimate the flying capacitor voltage. Compared with the traditional sliding mode observer, the proposed method can effectively eliminate chattering phenomenon, and has the advantages of faster convergence rate and higher estimation accuracy. Simulation and experimental results verify the feasibility and superiority of the proposed method.