Abstract: The calculation for the main cable curve during the design and construction control of suspension bridge is very important. Contrary to the fault of precise calculation theory which is difficult to converge in iteration of finite element and spends a lot of calculation time，taking account of the particularity of small sag on main cable of medium span suspension bridge，the theoretical basis on the transformation between beam element and cable element under different gravity stiffness was proposed. The feasibility of using beam element to simulate cable for calculating the curve of suspension was theoretically ensured. The basic principle and method of geometric nonlinear calculation for beam element was introduced，and the reliability of this method in this paper was proved by calculating the cable curve of the second bridge on Huihe River in Guzhen County. The actual result of the calculation indicated the advantage of rapid computing speed，stable calculating steps and easily converging for the simplified calculating method of cable curve of medium span suspension bridge．