The YL block in the Songnan low uplift of the Qiongdongnan Basin in the South China Sea has a water depth exceeding 1500 meters and is characterized by the development of fractured granite reservoirs. The high probability of encountering abnormal formation pressure makes it prone to complex operating conditions such as gas kick and loss circulation coexisting due to displacement during drilling, posing difficulties in predicting the multiphase flow pressure in the wellbore. The mechanism of gas kick and loss circulation coexisting due to displacement in fractured granite reservoirs has been revealed, and a gas-liquid two-phase flow model for these reservoirs has been established. A coupled multiphase flow model for the reservoir-wellbore system has been developed. Compared with the experimental data in the literature, the maximum relative error of gas influx velocity prediction has been reduced from 37.9% to 24.9%, and the maximum relative error of leakage velocity prediction has been reduced from 22% to 19%. The influence of fracture parameters, drilling fluid parameters, and other factors on the multiphase flow parameters in the wellbore are revealed. The results indicate that with increasing crack width and height, the decrease in overflow volume, leakage volume, and bottom hole pressures becomes more pronounced, leading to an increased risk of blowout. Conversely, as crack length increases, the decrease in overflow volume, leakage volume, and bottom hole pressures becomes less significant. With increasing drilling fluid density, flow rate, and wellhead back pressure, the decrease in overflow volume and bottom hole pressures is lessened, while leakage volume increase. However, with increasing drilling fluid viscosity, the decrease in overflow volume, leakage volume, and bottom hole pressures all become less significant. The coexistence of displacement-type leakage and overflow in complex working conditions can be identified by applying wellhead pressure, and can be handled by appropriately increasing the viscosity of drilling fluid.