Abstract: Based on the surface model of RRI, the calculation idea using large element-physical optics(LE-PO) is proposed to calculate the level of RRI. According to the assumption of LE-PO, the electric field integral equation is simplified. When the excitation source is plane wave, LP-RWG(Linearly Phased-Rao Wilton Glisson)basis functions are used to disperse the surface-induced current and the expression of scattering field from the tower of power lines was deduced. Related analysis example is used to analyze the calculation precision and the computing resource of LE-PO. As a result, the computing resources of LE-PO is much less than PO while the calculation precision of LE-PO is the same as PO.