Adding suitable quantity of polypropylene fiber is very effective for controlling plastic crack of cement mortar. During the experience stage, adding quantity of polypropylene fiber is 1.5 kg/m3, and crack rate of mortar is only 7.7%. Theoretical analysis is made for plastic crack mechanism of cement mortar, the results indicate that adding polypropylene fiber improves critical stress of crack expansion in mortar and notably reduces crack of cement mortar.