Effect of evaporation rate of water on the cement mortars plastic shrinkage cracking was investigated by changing temperature, humidity, light exposure and wind speed with different watercement ratio and the evaporation rate was calculated by simulation of mortar in small mold. The constitutive equation of plastic shrinkage cracking on evaporation rate of water as well as the constitutive equation of plastic shrinkage cracking on both evaporation rate and watercement ratio were established, which can be used to predict the plastic shrinkage cracking tendency of cement mortar.