The invention provides an automatic gear selecting and shifting mechanism of an AMT. Gear selecting and gear shifting operation can be achieved through one motor, and the space of an overall gearbox is reduced. The automatic gear selecting and shifting mechanism comprises a driving motor and a cam shaft, the cam shaft is sleeved with a plurality of gear slot cams, an embedding groove is formed in the center of the outer annular surface of each gear slot cam, each embedding groove is provided with a side convex embedding groove of which at least one portion faces a protrusion arranged at one end of the cam shaft, the non-side convex portions of each embedding groove are neutral gear position portions, and the radial angular positions of the side convex embedding grooves to the cam shaft are different; the automatic gear selecting and shifting mechanism further comprises shifting forks of which the number is the same as that of the gear slot cams, a side convex rod is arranged at the input end of each shifting fork, the side convex rod of each shifting fork correspondingly stretches into the embedding groove located at the corresponding position, the shifting forks are arranged in parallel and arranged on a same shifting fork shaft in a sleeving mode, and one end of the cam shaft is sleeved with a rotation input structure.