Taking non-grid-connected wind power system as the research object, a wind turbine testing platform is set up and a flow field measuring system is designed. On this platform, by changing the exciter current of the magnetic powder brake, the rotating speed of the wind turbine can be controlled with high accuracy. From the experiment results, it is found that the wake is developing in the opposite direction to the rotating wind turbine. And a conclusion can be given that the output power is getting larger when the axial velocity in the wake is smaller. These results provide base to establish the speed control model of the fixed-pitch variable-speed wind turbine, and reference to the optimization and design of non-grid-connected wind power system.