The invention relates to a diesel engine, especially a small single-cylinder diesel engine. Wherein, its burning room is a concave chamber reduced from up-to-down, while its bottom is arranged with taper cam; its inlet channel is a solid screw inlet channel with high vortex ratio; the depressurize stud of depressurize device is connected to the screw of depressurize axle to through the depressurize axle, to adjust the depressurize range of diesel engine. The invention improves the start property of diesel engine; reduce the fuel consumption and the pollution, to solve the problem that hard distribution of screw inlet channel caused by compact cylinder cover, to realize small diesel engine, and the problem of unstable depressurize range.