The invention discloses an oil-tank moisture detecting device. The oil-tank moisture detecting device comprises a detecting rod and an oil-trace-water precise analyzer. As multiple external connection openings including the sewage discharging opening, the sampling opening, the oil outlet and the like are formed in the bottom of an oil tank, and the movable mode is adopted in the original design intention of the detecting rod, one opening can be occupied to serve as an inserting inlet of a sensor. When the oil-tank moisture detecting device is used, a trace-water transmitter is connected with a data collecting device for following data transmission, then the moisture sensing sensor is directly fed into the oil tank through a tube to collect data, and the trace-water transmitter transmits the collected oil sample data to a data processor; as the oil-trace-water precise analyzer is movable, the data can be compared after being collected in real time, various kinds of oil sample data are stored inside the data processor, and the data processor compares the oil sample data collected in real time with the various kinds of oil sample data stored inside the data processor; when the oil sample data collected in real time shows that moisture exceeds the standard, the data processor controls an alarm apparatus to conduct alarming, and real-time detecting is achieved.