The unmanned helicopter is a special unmanned aircraft with a broad application prospect.However,the unmanned helicopter itself is a multivariable,nonlinear control system with strong coupling and many uncertainties.In this paper,the mathematical model of unmanned helicopter is presented,and a conventional sliding mode variable structure controller is designed at first.Then,a reaching law based sliding mode controller is designed to depress the chattering of the conventional sliding mode controller.Simulation experiment is carried out finally.The experimental results indicate that the reaching law based sliding mode controller has a better tracking effect than the other controllers on collective pitch angle control,and is effective for chattering attenuation,which can ensure the sliding mode dynamic quality.