The invention relates to an oil seal of a main feed pump. The oil seal of the main feed pump comprises an inner baffle, a middle baffle and an outer baffle. An inner groove is formed between the innerbaffle and the middle baffle, and an outer groove is formed between the middle baffle and the outer baffle. An oil return groove is formed in the bottommost portion of the oil seal. The oil return groove is an oblique surface which penetrates through the middle baffle and the inner baffle and becomes higher from inside to outside, so that lubricating oil flows out timely. After the oil seal runsfor eight months, oil leakage of the second main feed pump of a second Fuqing nuclear power unit disappears, the problem of lubricating oil leakage is completely solved, the reliability of equipment is improved, and safety operation of the unit is ensured.