The present invention discloses a planetary-gear-type speed variator capable of automatic gear shifting. The speed variator comprises a main shaft, a first planetary gear train, a second planetary gear train and a locking device, wherein the first planetary gear train and the second planetary gear train are arranged on the main shaft in turn, the second planetary gear train uses a first gear ring in the first planetary gear train as a planet carrier, and the locking device is connected with a second sun gear in the second planetary gear train. According to the above design, the structure is compact, two-gear speed output and stable gear shifting can be realized. The speed is changed at the first stage and output through the first planetary gear train, and then is changed at the second stage through coordination of the second planetary gear train with the first planetary gear train and the locking device and output by the second planetary train. The second sun gear is gradually locked by the locking device, so that the speed externally output by the second planetary gear train increases gradually, and thus the gear shifting stability is excellent. In this way, the pause discomfort brought about by abrupt changes of the speed during riding can be alleviated, the riding experience is optimized, and the riding safety is raised.