Aiming at the calculation of Radar Cross Section(RCS) of targets in Terahertz(THz) band, a parabolic equation model in cylindrical coordinates is proposed. Based on the general solution of electric field in cylindrical coordinate system, the excitation coefficients of each mode are decomposed by the orthogonality of trigonometric function. The parabolic equation method is extended to cylindrical coordinate system, and the fractional Fourier solution of parabolic equation in cylindrical coordinate system is obtained. On this basis, the target is equivalent to a series of surface elements or line elements, and then the parabolic equation is solved by the iterative recursive method of boundary conditions and field, and the scattering field of the series of surface elements on a certain cross section in the direction of propagation is obtained. The numerical examples show that this method can be utilized to calculate RCS of electrically large targets. Compared with the traditional parabolic equation method, it overcomes the limitation of scattering angle and has smaller calculation error. This research is of great significance for fast target recognition and imaging in THz band.