Abstract: The effects of polypropylene fiber under different length and different admixture content on the properties of alkali-activated slag cement mortar are studied. The results show that when the polypropylene fiber contents with 6mm and 12mm length are 0.04%~0.16%, workability of alkali-activated slag cement mortar is improved, but with the increasing of fiber content, the fluidity increasing extent decreases. When the content of the polypropylene fibers with 6mm length is 0.12%, the compressive strengths of the mortar at the earlier and later stages decrease, but their 28d flexural strengths increase by 25.5% and the ratio of flexural and compressive strength increase by 33.3% as well. When the content of the polypropylene fiber with 12mm length is 0.12%, the 28d compressive strength and flexural strength increase by 11.8% and 30.6% respectively and its 28d ratio of flexural and compressive strength also increases by 25%. By comparison, under the same admixture content, the polypropylene fibers with 12mm length have a bigger contribution to the compressive strength and the flexural strength of the alkali-activated cement slag mortar than those of polypropylene fibers with 6mm length.