Water shutoff is a commonly used method to mitigate the early breakthrough in horizontal wells. Gel is frequently used as an effective water shutoff agent in mature fields, especially for horizontal wells in recent years. However, the relevant water shutoff prediction model lacks the accurate physical description of the gelation phenomenon. Using the conventional model, which simply accounts for the gelation mechanisms, does not allow us to predict the horizontal wells performance correctly. In this paper, a newly coupled reservoir–wellbore model for horizontal wells gel water shutoff prediction is presented. A conventional gel simulator is used to simulate the gel injection process in the reservoir and then modified to predict the horizontal well performance after the treatment. The time-varying residual resistance factor model and viscosity model is developed to simulate the gel degradation process. Especially, the wellbore pressure drop calculation takes account for the non-Newtonian behavior during and after the gel injection. An explicit modular coupled scheme, which consists of reservoir modular and wellbore modular, is adopted to numerically predict the horizontal wells performance. The newly presented method not only simulates the gel injection process but also predict the water shut off performance in horizontal wells. A field horizontal water shutoff case prediction shows that the coupled modeling method can give satisfactory results to guide the water shutoff treatment.