The invention provides a novel engine in-cylinder combustion control method. The method includes the following steps of A, introducing combustion improvers to the periphery of a main fuel nozzle through a combustion improver guide pipe; and B, introducing a predetermined capacity of combustion speed reducers along a gas inlet channel through a combustion speed reducer nozzle in the high load section of an engine. During the working process of the engine, by introducing the combustion improvers through the combustion improver guide pipe or introducing the combustion speed reducers through the combustion speed reducer nozzle according to different working conditions, the working state of the motor can be controlled, and the working efficiency of the engine can be reasonably and efficiently managed. The risk of knocking caused by the continuous increase of combustion efficiency is avoided. Meanwhile, sufficient combustion is guaranteed as much as possible to reduce emissions under the premise of safe combustion.