The Specific surface area and pore structure are two important characteristics of shale. The specific surface area has an important influence on the adsorption characteristics and expansibility of shale. When measuring the specific surface area of shale gas, the result of specific surface area of shales is not so precise because of a large number of micro-nano pores. Based on the relationship between the specific surface area and space structure, the mathematical model is established, and the relationship between the specific surface area and pore structure of shale is discussed with two parameters, the number ratio and volume ratio. Results show that the microporous for specific surface area has important contribution to the shale, and small specific surface area is positive correlation relationship with pore volume ratio. This paper provides a theoretical basis for the study of shale gas adsorption characteristics and shale expansion characteristics.