In view of the complex factors affecting the water breakthrough of thermal horizontal wells and seriously affecting its production effect, numerical simulation methods are used to study the factors affecting the increase in water cut of thermal horizontal wells in the A oilfield, including the horizontal section length, daily liquid production, etc. Then, the water breakthrough mechanism of horizontal wells CSS is analyzed, and multi-factor influence sequence analysis is carried out by using orthogonal test method. On basis of the analysis of water breakthrough reasons of the high water cut horizontal wells in the A oilfield, the water finding method of comprehensive analysis of production dynamics and logging data is proposed, and the judgment criteria is set. Using the production data, pressure test, wellhead temperature and salinity of produced water from two typical high water cut horizontal wells ABA98 and ABA03 in target A oilfield, the water breakthrough reason are judged, and the source of produced water and the water breakthrough section combined with the logging data are analyzed comprehensively.