The utility model discloses an external constant pressure flushing device of a hydraulic mechanism, which comprises a motor, a supply reservoir, a discharge reservoir, and a high-pressure oil pump arranged on the output end of the motor, wherein an oil inlet pipe which is used to communicate with an oil discharge system of the hydraulic mechanism is arranged on the supply reservoir, the supply reservoir is communicated with the discharge reservoir through a filter, the discharge reservoir is communicated with an oil inlet of the high-pressure oil pump through an oil discharge pipe, an oil outlet of the high-pressure oil pump is communicated with a constant-high pressure system of the hydraulic mechanism through an oil supply pipe, a pressure gauze is arranged on the oil supply pipe, a pressure switch device is arranged on the oil discharge device, and is provided with an overtravel-limit switch. The flushing device can provide clean hydraulic oil to flush a hydraulic system in each movement of the hydraulic mechanism of a high-pressure circuit-breaker, and can completely clean away foreign matters in the hydraulic mechanism system of the high-pressure circuit-breaker.