For calculation models with symmetric characteristic, symmetry boundary conditions truncated by Perfect Electric Conductor (PEC) boundary and Perfect Magnetic Conductor (PMC) boundary are systematically analyzed, and a symmetric truncation method is proposed. Therefore the field values beyond truncation boundaries can be obtained so as to achieve the FDTD calculation on truncation boundaries. By analysis and calculation for dispersion characteristic of metallic-pole-planar SWS with floor structures, numerical results exactly consistent with global FDTD are obtained, thus feasibility and correctness of symmetric truncation method are verified. The results demonstrated by numerical simulation of dispersion characteristic of metallic-pole-planar SWS with floor structures are entirely equivalent to the results of global FDTD method. Calculation accuracy could be fully considered by the symmetric truncated FDTD algorithm. Numerical results show obvious advantages in both memory usage and computing time compared with the global FDTD method.