Abstract: With the technique of the physical property analysis,scanning electron microscope,casting slice,and high pres-sure Hg injection,the microscopic pore structure of Xishan-yao formation reservoir in Niuquanhu area of Santanghu oil-field was thoroughly analyzed and investigated. Therefore,pressure characteristic parameter of pore-structure P has been proposed,which can be used for expressing pore -throat characteristics. The research results indicate: the res-ervoir of this area had the characteristics of great mean (an average of 10. 23),medium sorting (an average of 2. 72),small median radius(an average of 0. 27 μm),minor expul-sion pressure (an average of 0. 45 MPa),medium median pressure(an average of 5. 22 MPa),low ejection efficiency (an average of 31. 05%),and high residual Hg content(an average of 61. 66%). On the basis of characteristic parame-ters of pore structure and the correlation analysis of it and physical property,capillary pressure curve of this area are divided into four categories. From class Ⅰ to class Ⅳ ,Pcd and PC50 gradually increase,pore-throat sorting changed for the better,and these all instruct that the pore structure and the percolation of reservoir are from good to bad.