The invention provides an automatic continuously variable transmission with a center shaft, which adopts a planetary gear train as a main structure and comprises a planetary gear set which is positioned in the housing of a right end cover at the side of a five-links; the planetary gear set comprises an internal gear, a planetary gear stand, a planetary gear, an external gear, the end cover of the planetary gear stand, a brake hoop and the controller of the brake hoop, wherein, the brake hoop is fixed on the right end cover and applies an adjustable frictional brake force on the internal gear. When road conditions are changed and cause different resistances, the internal gear generates different rotating speeds to automatically and continuously change the transmission ratio of the whole variable transmission, thus achieving the effect that the suitable riding condition of a rider can be realized at all times whatever the road conditions are. The invention has the advantages of simple and labor-saving operation and safe and reliable speed change, and breaks the conventional technological model of large fluted disc transmission with the center shaft, which uses a small fluted disc to realize the transmission effect of the large fluted disc. As to folding bicycles, the automatic continuously variable transmission with a center shaft is characterized by beautiful appearance and convenient use.