A numerical method based on Navier?Stokes (N?S) equations is developed to simulate the aerodynamic interaction of rigid coaxial rotor. Moving overset grid is employed to account for contrary rotations of two rotors. The validity of the method is verified by comparing with the experimental data. The aerodynamic performance and flow field characteristics of a coaxial rigid rotor in hovering condition are analyzed. Results indicate that the aerodynamic interaction can be divided into four aspects. The vortex induced effect is produced by the wake vortex of the coaxial rotor, which makes the aerodynamic performance of the upper rotor superior to the lower one. Periodic meeting of the double rotor leads to the blade loading effect, which produces periodic thrust fluctuations of the upper and lower rotors. The thickness effect makes opposite pulse fluctuations of the upper and lower rotor at meeting azimuths. The vertical blade-vortex interaction effect is caused by the collision between blade tip vortex from the upper rotor and the lower rotor blade, which affects the spanwise thrust distribution of the blade.