A dynamics simulator for Gas Circuit Breaker (GCB) with spring operating mechanism was developed. The spring stores mechanical energy. When opening and closing instructions are entered in the GCB, the solenoid opens the trigger, and mechanisms release the compression force of the spring at a high speed. A compact and light-weight mechanism is required for high-speed operation of the GCB. This means that the high-speed operation must be analytically investigated. We made a mechanical dynamics model of the whole GCB, including an FEM modal analysis. The dynamics simulator incorporates a solenoid electromagnetic field analysis and a gas pressure analysis of puffer chamber in the mechanical dynamics model. The operating characteristics of the GCB were analyzed with the simulator and measured in an operating experiment. The analytical results were in good agreement with the experimental results, and this proved that the simulator can be used to estimate the operating characteristics and appraise the stability of operation. The compact and light-weight GCB can be designed in short time by using the dynamics simulator.