In generators operation, the hydrogen purity is an important monitoring indictor. The change of the purity affects unit safety operation directly. Hydrogen purity of one #2 generator unit drop quickly. Determining how to find the main factors is the main objective. Primarily, it was analyzed that the main reason of hydrogen purity decrease was the air enter the sealing oil system. Though analyzing DCS history curve, as well as temperature detection test, channeling oil test, and checking the system pressure, finally there were two reasons, one is fill oil valve leakage fault, the two is the oil pressure difference of hydrogen side and air side in balance valve are too large, causing the oil channeling from air side to hydrogen side.