In order to calculate beam-column joints’ importance index in steel frame structure, an importance index calculation method was proposed. Firstly, equivalent resistance reduction coefficient was defined to quantify the influence of failure of joints on structure’s behavior. Then, equivalent failure probability coefficient was defined to quantify the failure probability of joints. Finally, based on Technique for Order Preference by Similarity to an Ideal Solution (TOPSIS Decision Method), considering equivalent resistance reduction coefficient and equivalent failure probability coefficient, a calculation method of beam-column joints importance index was proposed. The results of a four-story steel frame structure showed that this method could quantify joints’ importance and supply basis for inspection and appraisal of in-service steel structures.