To evaluate the collapse-resistant capacity of reinforced concrete (RC) frame structures, a quantitative criterion for mechanical hinge formation is proposed. Numerical simulation of RC frame joints is performed and verified using experimental results. A mechanical-hinge-based collapse criterion of structures is established. The criterion is used for the collapse analysis of a two-story reinforced concrete frame structure. The analysis results show that compared to the plastic-hinge-based criterion, the mechanical-hinge-based criterion works better for simulating the structural collapse and predicting the collapse resistance of RC frame structures.