The invention relates to a planetary transmission mechanism for an automatic transmission. The planetary transmission mechanism comprises an input planetary gear train, an output planetary gear train, an input shaft and an output shaft. The input planetary gear train comprises input planetary sun gears, input planet gears, input planet outer gear rings and input planet carriers, the input planetary sun gears are fixedly mounted on a transmission shell, the input planet gears are meshed with the input planetary sun gears, and the input gear outer gear rings are meshed with the input planet gears. The output gear train comprises output planetary rear sun gears, output common planet gears, output planet outer gear rings and output planet carriers, the output common planet gears are meshed with the output planetary rear sun gears, and the output planet outer gear rings are meshed with the output common planet gears. The planetary transmission mechanism has the advantages of simple part structure, reasonable design and convenience in control, the length of a transmission assembly can be reduced, the position of a clutch can be reasonably arranged conveniently, efficiency of the transmission can be effectively improved, and properties in aspects of responsiveness, riding comfort and the like are optimized.