The invention relates to an automatic transmission for a variable-speed bicycle, a bicycle and an automatic speed changing method. The automatic transmission comprises an automatic gearbox connected with a derailleur of the variable-speed bicycle, a torque transducer and an angle transducer, wherein signal transmission is conducted between the automatic gearbox and the torque transducer and the angle transducer. Compared with the prior art, the automatic transmission has the advantages that the defects of high weight and damping of an internal transmission rear hub are overcome, the automatic transmission can be applied to mountain bikes, and the structure is simpler compared with an internal transmission. By the adoption of the automatic transmission, fully-automatic speed change is achieved automatically according to the speed change rate and torque change of the bicycle during riding, hand operation is not needed, the purposes of saving effort and increasing speed are realized, and hand-operated speed change is replaced fundamentally.