The method for the calculation of radar cross section (RCS) of targets in terahertz (THz) band based on fast physical optics (FPO) algorithm is presented. The scatterer is divided into subdomains and the scattering characteristics of each subdomain are computed after the phase compensation. Through interpolation, phase correction and aggregation, the scattering characteristic of the whole scattering body is abtained, and the applications of FPO in THz band are discussed. Simulation results show that the targets′ RCS calculation by FPO in THz band can greatly improve the computational efficiency and save computing time in the premise of ensuring the accuracy. This research has an important significance for fast target recognition and imaging in THz band.