Abstract: The fast circuit breaker plays a crucial role in ensuring the safe and stable operation of the power system by swiftly removing short circuit faults. Aiming at a new type of hydraulic operating mechanism with fast response and high speed movement, we established a coupling relationship between the hydraulic operating mechanism, transmission system, and arc extinguishing chamber, and conducted a liquid-mechanical-gas co-simulation study. Moreover, we analyzed the influences of different operating powers on the motion characteristics of moving contacts, the buffering characteristics of hydraulic cylinders, and the reaction characteristics of arc extinguishing chambers. Based on this basis, the stress distribution of the transmission system was calculated, and an improvement strategy was proposed for parts with damage risk. The design scheme of the test prototype was determined. The research results show that the operating power significantly impacts the opening speed. The mechanism's piston rod side acting area is determined to be 3160 mm², increasing the opening speed to 13.63 m/s and shortening the mechanical separation time of the contact to 4.45 ms, meeting fast breaking requirements. However, with an improvement in the operating work of the mechanism, the peak of the mechanism buffer force and the peak of the pressure reaction force of the arc extinguishing chamber increase obviously, causing some transmission parts' stress values to exceed the standard. By optimizing the buffering performance of the hydraulic operating mechanism and the structure of the transmission parts, the transmission system which can meet the requirements of mechanical strength is designed. The research guides the development of the operating mechanism test prototype, whose test results meet the fast breaking requirements.