A computational study based on the unstructured embedded grid system and numerically solving the Navier-Stokes equations using Detached-Eddy Simulation (DES) model is carried out to investigate the flow field of a high-speed helicopter with rigid co-axial rotor and the aerodynamic characteristics of the tail-propeller with the influence of this field, and the aerodynamic interference effects of the rotor on the propeller are discussed, when the helicopter is in forward flight with different flow velocity, rotor lift and propeller pitch and position. Numerical results show that the rotor produces different directions of disturbance velocity to the space area below its rear, and the wake of the rotor and the propeller fuse and develop behind the propeller, thus the flow field structure is more complicated; under the influence of the rotor, the aerodynamic force of the propeller changes significantly, the propeller thrust, torque, and aerodynamic efficiency decrease in the usual situation, and there are obvious lateral force, pitch and yaw moments, and the unsteady aerodynamic phenomena become more significant. The effect of the rotor on the propeller is weakened with helicopter flow speed, rotor and propeller distance, and propeller pitch.