This paper based on thermal balance concept describes a calculation method of oil amount for thin oil lubricating industry gearbox.This paper introduces several thin oil lubricating modes normally used in industry gearbox,and describes the calculation method of oil amount for each mode.Especially,for the gearbox adopted the forced lubricating mode,thermal power loss and working experiences in factory are both considered by the calculation method,so the result of the calculation becomes more exact and believable.