The planetary gear assembly is a key sub-mechanical set of the Power Shift-Steering Transmission Device (PSSTD). With the help of vibration measurement, the diagnosis issue of the needle bearing failure in the planetary gear assembly is solved. For what, there are collision pulses appearing when the Elastohydro Dynamic Lubrication (EHDL) oil film break in the needle bearing, which can be monitored though high frequency component of the vibration signal. By the fault simulation experiments, the factor of the oil film break are researched. The conclusion is corresponding to the vehicle road test, to EHDL rule and to the bearing wearing style.