In order to make extended-reach drilling (ERD) of deepwater cost-competitive and obtain the maximum reservoir contact, dual-gradient drilling (DGD) technology is more and more used in offshore drilling. In this study, a method to predict the maximum measured depth (MMD) of the offshore horizontal well by DGD method in deepwater is presented. The prediction model is established based on the dynamic pressure balance of bottom hole. Two prediction models of offshore horizontal well's MMD by subsea mudlift drilling (SMD) method and hollow glass spheres (HGS) method are analyzed and established respectively. Moreover, effects of flow behavior index, consistency coefficient, drilling fluid density and ROP on these predicted MMDs with two DGD methods are analyzed. Finally, an offshore horizontal well in the South China Sea is analyzed. Some optimization measures are also put forward to obtain higher MMD and maximize reservoir contact. The study shows that there is a decrease of the predicted MMD as the flow behavior index, consistency coefficient and ROP are increased. Moreover, effects of drilling fluid density on the predicted MMD with different drilling methods are also analyzed. It shows that the ERW's MMD first increases and then decreases with the increase in drilling fluid density. It also shows that DGD method is conducive to expanding the MWW of horizontal well in deepwater. This work provides a practical tool for predicting the horizontal well's MMD of DGD in deepwater and achieving greater MMD. The study is significant to maximize reservoir contact and obtain good economic benefits of oil and gas development in deepwater.