In view of the position control accuracy and body chattering of small unmanned helicopter under large disturbance, an improved sliding mode controller (SMC) was proposed. Firstly, for the unknown parameters in the dynamic model of small unmanned helicopter named Align T-REX 300, three flight experiments were designed for the data acquisition, determining the comprehensive aerodynamic parameters. Then, the dual-channels control strategy with an improved SMC was designed to realized the helicopter hovering. The parameters ranges of SMC were presented by combining stability analysis, which can effectively suppress chattering. The model validation experiment shows that the aerodynamic parameters determined by the flight experiments have high fidelity. Moreover, the control simulations show that the steady state error of the step responses is less than 0.02 under the continuous large disturbance. Under the improved SMC with high control accuracy and weak tremor, the servos' control signal curves are smooth, which is more conducive for achieving the flight control of unmanned helicopter under the disturbance in actual engineering.