The hydration mechanism of the $3CaO{cdot}Al_2O_3-CaSO_4{cdot}2H_2O$ system in the presence of alkali sulfates has been investigated. The early hydration rate of $3CaO{cdot}Al_2O_3$ was accelerated by the addition of $Na_2SO_4$ and $K_2SO_4$. This is closely related to the formation of syngenite $(CaSO_4{cdot}K_2SO_4{cdot}H_2O)$, and the U-phase added $K_2SO_4$ and $Na_SO_4$ in the $3CaO{cdot}Al_2O_3-CaSO_4{cdot}2H_2O$ system, respectively. The formation of the rigid syngenite and U-phase structure led to rapid setting and decreases the sulfate content in the liquid phase of the hydrating cement to the extent that it cannot adequately retard the hydration of $3CaO{cdot}Al_2O_3$. In case of the alkali sulfate not added to the $3CaO{cdot}Al_2O_3-CaSO_4{cdot}2H_2O$ system, the ettringite was transformed to monosulfoaluminate immediately after the consumption of gypsum. However, when the alkali sulfates were added to this system, the ettringite did not transform to monosulfoaluminate immediately even though the gypsum was completely consumed. There was a stagnation period to transform to the monosufoaluminate after the consumption of gypsum because the syngenite and U-phase remained as the sulfate source.