Fuel transfer system is one of the key equipment which belongs to the fuel handling and storage system (PMC system) in PWR nuclear power plant. Its safe and stable operation is a prerequisite for fuel loading and unloading operation. During the first loading of a nuclear power plant in China, fuel transfer system had an overload problem of the conveyor trolley. The fuel loading operation was forced to stop on site, and the troubleshooting and treatment of this problem were carried out. This paper makes an in-depth analysis and research on this problem by using the Root Cause Analysis Method. After analyzing and troubleshooting the factors that may cause this problem one by one, we confirmed that the problem was caused by the over tolerance of the trolley straightness and the parallelism defect of some supporting rollers. Through adjusting the straightness of the conveyor trolley and the parallelism of the supporting rollers, this overload problem was finally solved successfully. All the results of the dry test, the 1m water test, the full water test and the fuel loading operation indicated that the conveyor trolley is in good working condition. The operating current of conveyor trolley is also stable within the design range.