The performance of helicopters can be significantly influenced by rotor speed variation. A performance analysis model is built to exam the influences of rotating speed, airspeed, take-off weight and flight altitude on the optimal rotating speed of a specific helicopter with a variable speed rotor. The Peters-He generalized dynamic inflow model and full aircraft trim model are introduced. The result shows 30% reduction in main rotor power is observed through rotor rotating speed reduction for the maximum take-off weight. For lower take-off weights or altitudes, the reductions in rotor speed and power savings are more apparent. The optimal rotating speed can cause a 20.5% increase in endurance and 8.5% in range by thoroughly consuming 400 kg of fuel. Setting thick airfoil at the inner side of blade can increase blade stiff ness, which leads to some increments in required power when speeding up, but has li ttle effect on the performance at middle and low speed, and no effect on endurance.