Nitrogen adsorption method was adopted to measure the pore of calcium based geopolymer, and pore structure of calcium based geopolymer characterized and the factors influencing the pore structure were discussed by analyzing the adsorption isotherm and the distribution of aperture. The results show that the pore structure of calcium based geopolymer is complicated, mainly composed of harmless pores and less harmful pores, as well as a few harmful pores. The pores mainly include cylindrical pores with open ends, slit shaped pores with parallel walls and ink bottle shaped pores with narrow neck and broad body. The diameter range of the main holes in pore is 350nm, which constitute 84.87% of the total pore volume, and 9191% of the pore specific surface area. The harmless and less harmful pores whose aperture is less than 50nm provide most part of the pore specific surface area and the total pore volume. The alkali activator and proportion of metakaolin are important factors that affect the pore structure of calcium based geopolymer.