The invention relates to the technical field of welding machining of reverse gear assemblies, in particular to a clamp of a reverse gear assembly, a welding system and a welding method. The reverse gear assembly comprises a shifting fork, a shifting fork rod and a fork opening frame. The clamp comprises a bottom plate, a first locating mechanism for locating the shifting fork, a second locating mechanism for locating the shifting fork rod, and a third locating mechanism for locating the fork opening frame. Both the first locating mechanism and the second locating mechanism are installed on thebottom plate and enable the shifting fork to abut against the shifting fork rod. The third locating mechanism is slidably installed on the bottom plate. The third locating mechanism slides, so that the fork opening frame abuts against the shifting fork rod. The shifting fork is clamped and fixed by the first locating mechanism, the shifting fork rod is clamped and fixed by the second locating mechanism, the fork opening frame is clamped and fixed by the third locating mechanism, and thus the shifting fork rod abuts against the shifting fork and the fork opening frame correspondingly. In thisway, the whole reverse gear assembly can be clamped and located through one clamp, welding is convenient, clamp replacement time is reduced, and welding efficiency is improved.