The invention discloses a compression and release type engine in-cylinder braking device. In a pair of cylinders with the phase difference of a 360-degree crank angle, a gas discharge mechanism of thecylinder A is provided with an oil cylinder device, a gas inlet mechanism of the other cylinder B is provided with an oil pumping device, the oil pumping device is controlled by the gas inlet mechanism of the cylinder B, engine oil of an engine is pumped into the oil cylinder device, then a valve rocker of the gas discharge mechanism of the cylinder A is driven to swing, thus a gas discharge valve of the gas discharge mechanism of the cylinder A is pushed to be opened, and release braking is achieved. According to the compression and release type engine in-cylinder braking device, switching of braking and non-braking can be achieved only by controlling on-off states of a switch electromagnetic valve, the requirement for a control circuit is low, working is stable and reliable, the failurerate is low, and the compression and release type engine in-cylinder braking device is especially suitable for being used for a multi-cylinder four-stroke engine with an even number of cylinders.