At present, the research on the water cut change of horizontal well is mostly concentrating on the reservoir with edge or bottom water, while there is little research on the water cut change of horizontal well in water injection reservoir.Aiming at this issue, the actual data of the oilfield is statistically analyzed.And it is found that the reservoir heterogeneity has the greatest influence on the water cut step rise for horizontal well.Then, a model for the numerical simulation mechanism is established to study the effects of reservoir permeability ratio and proportion of reserves in different permeability areas on the horizontal well water cut.Based on this, a theoretical chart of the dimensionless occurrence time of water cut step rise is established.It can accurately predict the occurrence time of water cut step rise.The results show that given constant low-permeability reserve ratio, the greater the permeability ratio is, the earlier the water breakthrough occurs,and the later the water cut step rises after the water breakthrough.Under constant permeability ratio,the smaller the ratio of low-permeability reserve is, the later the water breakthrough occurs,and the later the water cut step rises after the water breakthrough.When the permeability ratio is up to 1.5 or the relative low-permeability reserve ratio is up to33%, the horizontal well water cut will not increase significantly.Thenthe actual production data of oilfield BZ is used for verification analysis. The actual water cut change of horizontal well is basically consistent with the theoretical research,and the prediction for water cut step rise time is of high accuracy.The research results can be used to predict the change trend of horizontal well water cut in water injection reservoirand guide the understanding of flooding dynamic.