In a controller and method for preventing sending saturated control signals to an actuator, the controller receives parameters of controlling the actuator. The parameters include: a limit capacity value of the actuator, a predefined value which the status value of the plant needs to obtain, a ratio of the ideal control signal value to the limit capacity value, and a ratio of the actual control signal value to the limit capacity value. The controller calculates an ideal control signal value and an actual control signal value by applying a hyperbolic tangent function. And then the controller sends a control signal to the actuator according to the actual control signal value. The actuator drives a plant to move according to the control signal.