The utility model relates to a reversing mechanism used for a transmission overdrive. The reversing mechanism aims at a gearbox body without an upper cover and comprises a support guide block, a sway shifting block, an overdrive reversing guide block and an overdrive fork, wherein the overdrive fork is sheathed on a first fork shaft, the support guide block is sheathed on a second fork shaft, the support guide block is connected with a gearbox housing through a positioning device, the overdrive reversing guide block is fixed on the overdrive fork shaft, the middle of the sway shifting block is articulated on the support guide block, one end of the sway shifting block is inserted into a fork pocket of the overdrive fork and the other end thereof is inserted into the fork pocket of the overdrive reversing guide block. When engaging the overdrive, the overdrive fork shaft drives the overdrive reversing guide block to move and then drives the overdrive fork to reversely slide along the first fork shaft through the sway shifting block, thus realizing the purpose of engaging the overdrive.