The invention discloses a speed changer assembly suitable for a tricycle. An input shaft, an output shaft and a reverse gear idler shaft arranged inside a speed changer shell, the input shaft is sequentially sleeved with a forward gear driving gear, a synchronizer and a reverse gear driving gear, the synchronizer is connected with the input shaft in a sliding mode, the output shaft is fixedly sleeved with a main reduction gear, the reverse gear idler shaft is fixedly provided with a first gear and a second gear, the first gear is meshed with the reverse gear driving gear, the second gear is meshed with the main reduction gear, and the main reduction gear is meshed with the forward gear driving gear in a transmission mode; an engine stay cable support is arranged on the outer circumferential wall of the speed changer shell, and a gear shift stay cable support is arranged on the outer wall of one end face of the speed changer shell. The synchronizer is applied to the speed changer assembly suitable for the tricycle and is improved according to characteristics of the tricycle, and therefore the technical problem that the gears in the speed changer assembly suitable for the tricycle are seriously damaged for a long time is solved effectively; by optimizing the structures and layout of the stay cable supports, the overall structure is more compact and reasonable.