Abstract: The sidelong edge is an important composition part of the rotary blade. In order to meet design requirement of sidelong edge and to obtain right expansion graph of the edge, the expansion equation of sidelong edge is derived in this paper according to manufacturing procedure of rotary blade. With this equation, the sidelong edge of rotary blade can be developed into the plane of lengthwise edge and the expansion graph of sidelong edge can be calculated. The expansion graph of sidelong edge can also be simulation with the eccentric circle. The result shows that the eccentric circle simulation for sidelong edge expansion graph only has very small deviation. This makes it easy to process and check sidelong edge of rotary blade.