The Radar Cross Sections(RCSs) of target radar in Terahertz band are simulated and compared in two sets of electromagnetic computing software, FEKO and CST. The RCSs of the same perfect conducting sphere are calculated in 1 GHz-500 GHz, thus the RCS characteristic curve is obtained, and compared with the theoretical values. The comparison results indicate that the simulation performances of FEKO and CST in Terahertz band are similar. Then the RCSs of perfect conducting sphere, cylinder and rectangular plate in THz band under different aspect angles are calculated by using FEKO and CST. It suggests that FEKO and CST exhibit different performances with different targets and suitable software should be chosen according to different targets.