Forward aerodynamic performance of coaxial rigid rotors mainly depends on flow velocity and density in working condition, and airfoil configuration, chord distribution and twist distribution in aerodynamic configuration parameters. Synthesis effects of rotor aerodynamic configuration parameters reflect on the value of lift-offset. It facilitates designing coaxial rigid rotors suited to fly in high speed to investigate influence principles that flow velocity and lift-offset impose on forward performance. Therefore, forward flow-field of a 4 m diameter coaxial rotor composed of two 2-blade rotors are numerically simulated by solving the Reynolds-averaged Navier-Stokes(RANS)equations. Aerodynamic forces at different advance ratios and performance with different lift offset are obtained. The results show that thrust distribution on the blade spanwise is more reasonable, and thrust center moves toward middle blade part which could offload the blade tip. The lift of rotor is mainly provided by the advancing side of each rotor and large lift-offset leads to shock wave stall, which has adverse effect on high-speed forward flight.