Braking characteristics of piston-dashpot system in a hydraulic operating device for circuit breaker was investigated by the measurement and numerical analysis. Braking force against piston motion is generated in the use of pressure drop of gap flow through annular dashpot formed at the end of the piston. One-dimensional flow and motion of piston-dashpot system were modeled and analyzed dynamically to evaluate this force and piston displacement. Formulas of pressure drop in dashpot gap flow which distinguishes turbulent flow from laminar flow by Reynolds number were used in the analysis. A piston moving part was modeled by two particles model system in consideration of elastic link mechanism. The validity of the hydraulic and dynamic analysis was confirmed by the comparison with experiments of a testing device. The effect of dashpot dimension such as annular gap, dashpot diameter, dashpot length and the number of gap stage on braking characteristics was surveyed and the range of effective braking characteristics was showed. These results are useful for the proper design of dashpot dimension of hydraulic operating device.