Abstract: It is challenging to perform horizontal well drilling in deepwater shallow water hydrate sediment. Due to the phase equilibrium characteristics of natural gas hydrates, the temperature and pressure disturbance resulted from drilling fluid invasion can easily cause the decomposition of hydrates around the well, thus increasing the risk of borehole instability. To study this problem, this paper proposes a wellbore-reservoir coupling model. The model considers the dynamic changes in both mud cake porosity and permeability and reservoir mechanical properties, hydrate phase changes, and the heat-mass transfer coupling of wellbore multiphase pipe flow and reservoir multiphase seepage. The comparison with experimental data indicates that the model can effectively characterize the changes in parameters such as temperature and pressure around the well caused by the invasion of drilling fluid in hydrate reservoirs. Based on the drilling data of Well Site SH2 in Shenhu area of South China Sea, the paper analyzes the characteristics of drilling fluid invasion and the risk of borehole instability in the long horizontal section of hydrate reservoirs. The results show that with the extension of the horizontal section, the temperature and pressure in the wellbore annulus gradually go up, and the invasion depth of drilling fluid and the risk of borehole instability increase. The horizontal well location is deployed in the reservoir with high hydrate saturation, which helps alleviate the invasion degree of drilling fluid and reduce the risk of borehole instability. Further, the paper simulates the effect of mud cake, different drilling fluid inlet temperatures and salinities on the borehole stability at the toe of horizontal well. The results demonstrate that mud cake has a more obvious effect on reducing the drilling fluid invasion depth and the yield radius around the well in the low hydrate saturation area. The changes in drilling fluid salinity mainly affect the yield conditions around the well. It is suggested to use the drilling fluid with lower salinity, so as to reduce the yield area and yield degree around the well in the drilling process of hydrate formations. Under the calculation conditions in this study, the salinity of the drilling fluid lower than 4.2% is more conducive to borehole stability. Under the same calculation conditions, the critical drilling fluid inlet temperatures at the toe of well locations ① and ② are 22.39℃ and 22.24℃, respectively. To avoid yield around the well in the drilling process of hydrate formation, the drilling fluid inlet temperature should be lower than the critical yield temperature.