Planetary gear train is the core of the automatic transmission（AT）and is the key part to decide the performance of the AT.Its efficiency varies with different structure types.Based on the planetary gear train of CX31

the structure characteristic is analyzed

the transmission ratio and transmission efficiency are calculated.On the condition that the speed ratio distribution remained largely unaltered

the characteristics and transmission efficiency of another typical configuration are studied.And its alternative is analyzed.It is provided some theoretical and data support for automatic gearbox independent design of heavy vehicle.