The thrust bearing of drive shaft assembly is the most critical part in the downhole motor assembly. Because the working condition in the downhole motor is very abominable (drilling fluid lubrication, large load of bearing group with alternating load), the working life of the thrust bearing is much shorter, which makes drive shaft assembly the weakest link of the whole downhole motor. Therefore, adopting the oil storage to compensate the sealed drive shaft assembly will promote transmission efficiency, and greatly improve the life of the drive shaft.