This study investigates the influence of the precursor modification on the retardation of flow loss in alkali-activated slag (AAS). The blast-furnace slag precursor was modified by using a modifying solution incorporating NaOH or waterglass (Na2SiO3) to suppress the dissolution rates of the main reaction components such as Ca, Si, Mg, and Al. The influence of the Na2O/slag and silicate modulus of the modifying solution on the characteristics of the modified precursors was investigated in parallel with the leaching mechanism of Ca, Si, Mg, and Al from the modified precursor. The leached concentrations of Ca, Si, Mg, and Al from the modified precursor were less than those from the unmodified precursor. AAS paste was then produced from the modified precursor, and the flow property and compressive strength was investigated. AAS paste produced from the modified precursor maintained the flowability for 2 hours and had a compressive strength of 20 to 50 MPa (2900.8 to 7251.9 psi) at 28 days, while the flow of that produced from unmodified precursor was rapidly decreased within 1 hour and the compressive strength was approximately 65 MPa (9427.4 psi).