The invention relates to a six-gear automatic speed changer based on a planet mechanism. The six-gear automatic speed changer comprises a hydraulic torque converter, an input shaft, a planet mechanism assembly, clutches, a brake, a one-way clutch, a first transmission gear, a second transmission gear and a third transmission gear, wherein the planet mechanism assembly comprises three groups of planetary gear trains, a first planetary gear train, a second planetary gear train and a third planetary gear train are distributed on the input shaft in sequence, and are all positioned between the hydraulic torque converter and the clutches, the hydraulic torque converter is connected with one end of the input shaft, a guide wheel of the hydraulic torque converter is fixed on a shell of the speed changer, a first clutch, a second clutch and a first sun wheel are connected with the other end of the input shaft, the first clutch is simultaneously connected with a second sun wheel and a third sun wheel, and the second clutch is connected with a second planet carrier. According to the six-gear automatic speed changer based on the planet mechanism, the structure is simple, the design is reasonable, the brake and the clutches are assembled simply, the cost and the machining difficulty of the parts are low, the assembly is simple, the cost is low, and the use and the promotion are convenient.