The invention relates to a drilling tool, in particular to a hydra-mechanical reducing bit broaching while drilling. The hydra-mechanical reducing bit broaching while drilling is capable of continuously caving in coal beds after a drilling-in process and in a bit withdrawal process, and comprises an upper connector, a shell, a core pipe, a mandrel, a bit, a piston system, a drilling fluid flowing direction converting system and a cone arm assembly broaching while drilling. The hydra-mechanical reducing broaching bit while drilling is reasonable to design and is capable of achieving the same functions of a normal bit in the drilling-in process and enabling a cone arm to stretch under the action of the piston system, the drilling fluid flowing direction converting system and the cone arm assembly broaching while drilling during bit withdrawal, so that a broaching and caving process is achieved.