Abstract: A point of view that the carbon fiber has composite structure is proposed. The relationship between the practical elasticity coefficient of carbon fiber and the crystallinity, micro-fiber orientation and pore conditions was researched based on the micro mechanics theory for mechanical properties of composite materials, the continuous fiber reinforced single plate principle and the properties of microstructure of carbon fiber. The analyzing and calculating methods were obtained, and the main factors which affect the elasticity coefficient were found. A innovative theoretical analysis method and the definite calculation formula were given. The obtained theoretical results are consistent with the experimental results. It is revealed that the pore density, the fiber size and orientation, the crystallinity determine the elasticity coefficient of carbon fiber, which provides theoretical basis to improve the carbon fiber production process.