Abstract: The conventional resonance calculation method known as equivalence theory generates the effective resonance cross section by setting up the equivalent relationship between heterogeneous and homogeneous system. It is widely utilized in lattice physics calculation. Several approximations involved in equivalence theory including spatially constant flux approximation, rational approximation of the neutron escape probability and narrow resonance approximation cause considerable deviation of effective cross sections compared to those generated by rigorous method, such as ultra-fine-group method and Monte Carlo method. The deviation caused by each approximation was analyzed using resonance calculation codes based on equivalence theory and ultra-fine-group method. Results show that spatially constant flux approximation and narrow resonance approximation are the main source of the deviation. A new derivation of effective cross section by equivalence theory was proposed to improve the conventional method and calculation flow. And the deviation caused by conventional equivalence theory is corrected systematically by the new method. Because the new method is simple and efficient, it is convenient to be implemented into the existing codes to utilize the equivalence theory.