The invention discloses a compression releasing engine cylinder inner braking device. The device comprises a camshaft provided with multiple exhaust cams; the camshaft is axially provided with an oilpassing cavity; the oil passing cavity communicates with a set of oil supply device only through a set of rotary oil inlet joint device; the exhaust cams are radially provided with mounting holes; sliding plunger mechanisms are arranged in the mounting holes; and a throttling device is arranged in the oil passing cavity. The device improves the engine working reliability, and accelerates the oil injecting speed of the oil supply device to the oil passing cavity of the camshaft after an engine enters a cylinder inner braking state, so that the sliding plungers are quickly projected to form braking projections, the cylinder inner braking response time is shortened, the structure is simple and compact, and the cost is low.