Which factor for a sodium silicate-based activator influences the setting time of alkali-activated slag (AAS) paste is investigated. Several factors, such as the pH value of the activator, the alkali modulus (AM) and the alkali activator dosage, were evaluated when the liquid/slag ratio was kept constant. It was found that the pH value and the AM have a strong relationship, but they have no distinct relationship with the setting time. The activator dosage, which is defined as the sum of the SiO2 and Na2O concentrations, shows a significant trend with the setting time. It was also found that the influence of SiO2 on the setting time is more apparent than that of Na2O. Increasing amounts of SiO2 decrease the pH value of the activator and increasing amounts of Na2O increase the pH value of the activator. Phosphoric acid, used as a retarder, was found to have a strong retarding effect.