The flowfield of ?2 m rigid coaxial rotor in hovering state were measured in wind tunnel based on particle image velocity (PIV) technique. The test condition with rotational speeds of 1 100 r/min and 1 860 r/min, collective pitches of 8° and 10° were measured on the test rigid coaxial rotor stand. Meanwhile, the isolate upper rotor under the same condition were measured for comparison. The trajectory of tip vortex and the distribution of velocity vector were obtained. Furthermore, the aerodynamic interaction between the upper rotor and the lower rotor were investigated, and the "secondary contraction effect" of the vortex wake of the upper rotor was observed when it passed through the disk of the lower rotor.