The occurrence of water breakthrough is inevitable in the development process of bottom water reservoirs，and it is difficult to control the water cut after water breakthrough for the multilateral wells. Therefore，it has great importance to prolong the water breakthrough time. There are a few studies on the water breakthrough laws for the horizontal wells，but no studies for the multilateral wells. Relationship between the water breakthrough time and related parameters was obtained by numerical simulation so as to provide theoretical foundation for parameter adjustment and longer water breakthrough time.Experiment results indicate that the water breakthrough time has positive correlation with the water avoidance height，the control area for single well，the area and position of interlayer，the horizontal permeability and the three multilateral well parameters including branch length，branch angle and branch number. On the other hand，the water breakthrough time has negative correlation with the pressure difference，the fluid production rate，the vertical permeability and the ratio of oil and water. The observation of the water ridge morphology of the bottom water reservoir developed by herringbone wells and radial multilateral wells indicates that the main water breakthrough point of the herringbone well is at the intersection of the main wellbore and the branches，and the shape of the water ridge is related to the shape of the branch. So the intersection may be the position of bottom water breakthrough. The heel of the radial multilateral well may has earlier water breakthrough than the toe. While the first water breakthrough point for the radial multilateral well is the intersection of the branches，and punctiform water breakthrough occurs at first，followed by the planar water breakthrough. The shape of the water ridge is related to the shape of the branch. Therefore，the position of water breakthrough is at the heel of the radial multilateral well，which is different from that in the herringbone well.