Abstract: Measuring the water cut of the crude oil with automatic on-line sampler rather than by the conventional method of representative sampling can increase measurement accuracy, and eliminate the inadequate representativeness existing in manual sampling for turn-over measurement. Introduced are the technical parameters, structural features and operating principles of the automatic on-line oil sampler of ZDQY-SB Ⅱ type. Comparisons made between the results in actual applications of the sampler and that indicated in the intelligent water cut meter show that the difference between them are neglectible. Therfore, it is concluded that this type of instruments is reliable in the measurement of water cut in crude oil.