The particle image velocimetry（PIV）experimental technology is used in the experiments to measure the co-axial rigid rotor blade vortex wake and flow field around the blade. From these experiments, the blade vortex trajectory and the blade tip induction velocity distribution of the single/dual rotor are obtained. The influence rule of the rotor forward ratio on the offset angle of trail boundary is revealed. The influence mechanism of the strong radial flow on the flow field around the blade is researched. The results show that the strong radial flow can enhance airflow adhesion, delay airflow separation and stalling. Besides, the trailing edge separation piont is dalayed about 18% at the 0.64 forward ratio compared to 0.53.The greater the forward ratio of the rear side reverse flow region is, the stronger the reflow intensity is. In hover, the upper-rotor vortex of the dual rotor vortex contract fastest, the single rotor vortex contract second, and the lower-rotor vortex of the dual rotor contract slowest. The measurement result is reasonable and reliable, which lays the foundation for theoretical research of rotor flow mechanism and improvement of CFD analysis accuracy.