To improve the accuracy of helicopter performance calculation in conceptual design phase, an analytical method of helicopter flight dynamics is proposed for conceptual design. In the method, the attitude angle of fuselage is set before trim, and in the trim process the attitude angle is set to be equal to the set v alue by changing the lengthwise position of the horizontal tail and the vertical position of the tail rotor. So when the overall design parameters vary, the effect of fuselage attitude change on the flight performance can be ignored, thus analyzing the influence of the overall design parameters on t he flight performance and optimizing the overall design parameters in the conceptual phase.