Abstract: Based on the engineering reality, a calculation model of the rotor unsteady aerodynamics of the small coaxial helicopter in hovering state was set up. An airfoil unsteady aerodynamic model was established by introducing a set of Leishman-Beddoes semi-empirical indicial response formula. The induced velocity of the aerodynamic interaction between upper and lower rotors of a coaxial helicopter was set up by bringing in an interferential factor in the dynamic inflow model, the Runge-Kutta method was used to obtain the numerical solution of the rigid blade flapping angle. According to the calculation and analysis, the thrust and torque response to a rapid collective pitch increase in hovering state was gotten, and the course response differences between the semi-differential and full-differential direction control modes was compared.The results would be a necessary preparation for the research of coxial helicopter-s maneuvering flight.