In order to achieve high speed and effective simulation of antenna-radome system, according to the shortages such as step-itera-tions, small-step searching and complex window function transformation in conventional algorithms, in this article, the concept of far-field effective viewing angle was introduced, only small extent viewing angle near extreme point had been considered. The solving of far-field pattern and BSE was separated, far-field pattern was fitted by least square method curve fitting method, and the BSE can be solved by fit-ting function directly. The simulation result demonstrates that the independence on single point's precision is reduced; the new arithmetic's efficiency is highly improved without reducing the accuracy.