The Qiongdongnan Basin is categorized as a medium-porosity and low-permeability reservoir, making it susceptible to damages such as hydration and water blockage. The invasion damage type and mechanism caused by the deep water drilling fluid used in the current well series are not well understood. Additionally, traditional methods for assessing drilling fluid damage suffer from significant errors and an inability to quantitatively measure the extent of damage directly. Therefore, a method combining drilling fluid contamination experiments, scanning electron microscopy (SEM) analysis, and computed tomography (CT) was designed to analyze the solid-phase and liquid-phase invasion damage of drilling fluid to reservoirs. The results demonstrate that the compatibility between the deep water drilling fluid and formation water is good. The water blocking damage rate ranges from 19.8% to 31.4%, with water blockage being the primary liquid-phase invasion damage. SEM scanning of core samples reveals poor pore connectivity, and EDS testing indicates elevated levels of Ba²⁺ and Ca²⁺, suggesting that solid-phase invasion damage is mainly caused by weighting agents. After distillation water backflow, CT scanning of the core samples shows a slight increase in porosity, indicating that solid-phase plugging is not easily removed through natural backflow. To optimize the reservoir protection performance of the deep water drilling fluid, the development of a filtrate reducer and the optimization of weighting agent particle size distribution are proposed. According to the ideal filling theory, the optimal ratio of 1 000 mesh CaCO₃, 600 mesh CaCO₃, and 200 mesh CaCO₃ is determined to be 5∶11∶9. Following optimization, the filtrate loss of the system is significantly reduced, the median particle diameter of solid-phase particles is around 90 μm, and the compactness of the filter cake is notably improved. The permeability recovery value increases by 12.1% to 19.68%, offering significant guidance for optimizing the reservoir protection performance of drilling fluid in this area.