Geopolymer is a kind of green and low carbon cementing material, and the study of the cementation process of geopolymer is essential for engineering application. In order to investigate the mechanism of geopolymer cementation, five groups of geopolymer mortars with different mixture proportion (slag: fly ash) were placed in a special device and monitored them in real time by three ultrasonic inspection methods. The initial and final setting time of the geopolymer mortar were characterized by energy reception ratios and verified with the results of the penetration resistance method. The results demonstrated that the setting rate of geopolymer mortar slows down with the decrease of mixture proportion. The ultrasonic transmission method could realized the whole monitoring of mortar. The mean error of initial setting time and final setting time of ultrasonic transmission method was 7.9% and 6.6% respectively. The reflection method has high accuracy in determining the initial setting time of mortar, and its mean error was 2.8%. The ultrasonic guide wave method could realized the whole monitoring of mortar. The mean error of initial setting time and final setting time of ultrasonic transmission method was 3.3% and 2.5% respectively. Among the three methods, ultrasonic guided wave method is more suitable for monitoring the coagulation process of geopolymer mortar.