Effects of different dosage(by mass, the same below) of polyacrylamide(PAM) thickener on the shrinkage cracking properties of cement mortar in plastic phase were studied by measuring the water evaporation rate, plastic tensile strength, plastic shrinkage stress, crack resistance index and plastic shrinkage weighted cracking value. The results show that PAM has a good effect of reducing the plastic shrinkage cracking of cement mortar. With the increase of PAM dosage, the evaporation rate of cement mortar tends to decrease at high wind speed. When the dosage exceeds 008%, the evaporation rate increases. The plastic tensile strength and plastic shrinkage stress decrease with the increase of PAM dosage to some extent and no longer decrease after reaching the critical dosage(006%). The crack resistance index increases with the increase of PAM content, and the plastic shrinkage weighted cracking value shows a significant decreasing trend.