Abstract: This paper introduces the work principle of PDH-series water-cut meters, which is used to monitor the variation of water content during oil custody transfer metering in-line. Application result shows that this PDH-series water-cut meter is able to effectively control and reflect transition variation of water content of oil during oil custody transfer metering, which lowers down metering errors induced by the variations of water content, decreases the loss of transportation shortage and increases the accuracy of custody transfer metering of oil.