A compression release engine in-cylinder braking system, comprising a valve mechanism, an oil cylinder device, an oil pump device, and an oil supply device. The oil cylinder device and the oil pump device of each air cylinder communicate with each other through a pressure transmission oil circuit, which communicates with the oil supply device through a low-pressure relief valve. An air release valve is arranged at the high end of an oil circuit system. During in-cylinder braking, the air release valve is closed, an electromagnetic reversing valve is energized, engine oil having a pressure of P1 is supplied to the pressure transmission oil circuit, and a cam abuts against and pushes the oil pump device to pump high-pressure oil to the oil cylinder device, so as to push a rocker arm to open a valve, thereby achieving in-cylinder braking.