The problem of drilling fluid leakage in Nanpu No.2 and No.3 structure in Jidong Oilfield is serious. In order to understand the leakage mechanism in this area, the micro structure analysis and physical and chemical properties test of the rock samples were carried out. The mechanism of drilling fluid leakage in this filed was analyzed by combining with the field statistical data. The results show that the lithology of the layers in Nanpu No.2 and No.3 structures is mainly hard and brittle mudstone, which has weak water sensitivity and shows hard and brittle characteristics. The microfractures in the rock are developed, and the drilling fluid intrudes along the fractures, resulting in the weakening of the overall cementation strength of the rock, which is prone to tensile fracture along the fracture surface. According to the field data, the fractures in rock is the main factor of drilling fluid leakage. Drilling fluid intrudes into the formation, resulting in the decrease of mechanical strength, causing rock damage and leakage. In view of the complex leakage reasons of Nanpu No. 2 and No. 3 structures, the Countermeasures for different leakage reasons are proposed. The research results provide a technical reference for understanding the mechanism of drilling fluid leakage and reducing drilling cost.