Abstract: Caves and fractures are the main room for preserving and transporting in fractured-vuggy reservoirs of Tahe Oilfield. With strong heterogeneity, the fluid flow mechanism of caves varies a lot from that of the general sandstone reservoirs. The water content of a kind of reservoir rises suddenly while producing, and it to some extent declines and then rises again.The phenomenon may take place 2 or 3 times. To explain it, a new hydrodynamic model is established, based on the tubing flow theory. The development features and the characteristics of water cut change are also simulated by applying the difference calculating method. Results show that the water breakthrough time depends on the crack width. The water cut fluctuates with the water breakthrough time, because fractured-vuggy systems are simultaneously connected by oil wells.