There are many problems in the four-tank system, such as the model's uncertainty, slow dynamic performance,and serious coupling.Aiming at these problems, this thesis designs a fractional-order sliding mode control algorithm to control the water tank's liquid level. Specifically, the traditional integer-order sliding mode surface and the approaching rate are improved to obtain a fractional-order sliding mode surface and approaching rate with more flexible control effects. Moreover, Lyapunov Theory is applied to theoretically analyze the asymptotic stability conditions, application range, and characteristics of the fractional sliding mode controller, and the algorithm design method is proposed. Through the comparison with the fractional PID controller, the simulation experiment reveals that the fractional sliding mode controller possesses good dynamic performance for the expected value of the given liquid level, can effectively reduces the overshoot in the control process, and has good robustness to the perturbation of system parameters.