This paper reviews the principle of power production, and analyses the influnece on aerodynamic load and output power due to the variation of pitch angle. For three-bladed upwind horizontal axis wind turbine, the blade pitch control is used primarily to adjust the power coefficient and obtain the optimal power at high wind speed, but it also make aerodynamic load, such as edgewise, flapwise and torsion moment, change during variable pitch control. Hydraulic mechanism is used in a process requiring large driving forces and torques, fast response and high stiffness. Therefore, the variable pitch mechanism is operated using electro-hydraulic proportional technology in this paper. Simulation results are presented and analysed to show that aerodynamic load and output power are sensitivity to pitch angle for wind turbine blade. These works lay foundation for the further studying of individual pitch, power control, fatigue and dynamical stability for wind turbine.