In this paper, a high-order symplectic finite difference time domain schemes is constructed for Hamilton system, using the symplectic Runge-Kutta-Nystrom (SYRKN) method. On the basis of fixed phase space and total energy in the Hamilton system, a formula of iterative algorithm is proposed which can be used to calculate the radar cross section of a metal cylinder. Calculation results have shown that the SYRKN method is accurate and fast.