Abstract: Water breakthrough is one of the key problems occurred in staged fracturing or water injected horizontal wells in Changqing Oilfield. After water breakthrough, water cut increases rapidly and production capacity decreases abruptly, which seriously restricts the efficient development and large-scale application of horizontal wells. Water exploration technology is a prerequisite for controlling water and stabilizing oil production. Conventional technology has a very long period of water exploration (60-80 days) and high costs (800 000 Yuan/time for production profile test) due to the number of test stages, which cannot meet the large-scale development of horizontal wells in Changqing Oilfield. A downhole sampler has been developed by innovation, which meets the 75 ℃ and 40 MPa downhole sampling conditions in indoor evaluation tests; the sampling timing and manner have been further established according to the varying pattern of production performance in field test wells, and hence a full set of downhole sampling and water exploration technology has been developed, which shortens the water exploration period to 6-7 days and the cost to 200 000 Yuan/time. This shows that downhole sampling and water exploration technology has been greatly optimized in terms of field test and economic benefits, and provides an economic and effective technical means for water exploration method in horizontals in Changqing Oilfield.