There are abundant geothermal resources in Fengheying which is located at the depression of Central Hebei, and the hot water reservoir is mainly JXW, belongs to carbonate crack and karst reservoirs. There are accidents such as leakage in hot water reservoir, clay swelling and collapse in the drilling process. Based on the analysis on the geological characteristics of the reservoir, the performance evaluation is made on the drilling fluid system. The experiment shows that the original drilling fluid system had poor wall building and leakage blocking properties. According to the geological characteristics of the reservoir and the mechanism of accidents, the performance of the original drilling fluid is optimized. By adding 5% superfine calcium carbonate and 5% potassium chloride into the original drilling fluid system, the performance of new drilling fluid is greatly improved with good rheology, strong inhibition, good blocking capacity and low filtration. The damage to Fengheying geothermal reservoir can be reduced by the optimized drilling fluid system.