B Oilfield is a typical thick porous edge-water carbonate oilfield in the Middle East. The oilfield has a production history of over 40 years. In order to achieve better development effect, horizontal wells have been implemented on a large scale in the past decade. In the development process, affected by strong planar and vertical reservoir heterogeneity, some horizontal producers start to produce with relatively high water cut, which causes a higher oil decline rate. The large oil–water transition zone and strong reservoir heterogeneity of the reservoir result in complex water breakthrough mode. Meanwhile, there is no condition for the horizontal producers with ESP to perform saturation logging as well as PLT tests, thus it is with high necessity to come up with a reliable method to effectively diagnose the water breakthrough types of the horizontal producers, which aims to provide supportive guidance for water control operations as well as water flooding optimizations. This research establishes an innovative and integrated method to systematically diagnose the water breakthrough type of horizontal wells and is capable of finding out the water production intervals. The integrated method consists of 4 steps. In the first step, the classic WOR and WOR’ diagnostic method is applied to preliminary identify the possible water breakthrough type. The next step is to compute the respective water breakthrough time through proper reservoir engineering methods and compare the breakthrough time under different circumstances. In the third step, history matching of water cut by numerical simulation is conducted based on the static and dynamic information obtained, the comparison results of the error rates in different matching scenarios are referred to determine the realistic water breakthrough mode, and the main water production intervals are figured out based on reservoir simulation. Finally, combined with the regional geological understandings, the entire water breakthrough process of the target well is depicted. In this paper, a typical water-out horizontal producer is taken as an example to detailed demonstrate the entire diagnostic procedure. The understanding on water breakthrough mode of the horizontal wells in this method is applied to guide the actual water control treatments and water flooding adjustment of the field and has efficiently improved the regional development effect.