Spring Operating Mechanism (SOM) is a dynamic system to open and close the circuit breaker in a voltage controlling system. For a high-speed action of opening and closing within a few mili-seconds, a SOM consists of many links, joints, chains, and cams. Thus, various dynamic characteristics are occurred, especially large contact forces between the cam and the roller, the shaft and the stopper. To save time and money for a new design of SOM system, analysis of the mechanism is necessary. In this paper, a multibody dynamic analysis and test technique was applied for a SOM to predict, estimate and validate forces occurring during the operation. For the multibody dynamic analysis, the ADAMS program was employed for 145kV circuit breaker with a SOM. For an accurate modeling, several components were sequentially added and the reliability of modeling was validated through the comparison with test data of opening and closing.