The high-fluidity slag and desulphurization gypsum composite alkali material with (HSD-AAM) as precursors is one of the new building materials commonly used in the restoration and reinforcement of structures. In order to ensure fluidity, HSD-AAM usually uses a high water-binder ratio, which affects its early strength and thus limits the application of HSD-AAM. To solve this problem, the impact of the amount of nano-silica (NS) on the fluidity, setting time, flexural strength and compressive strength (1d, 3d, 7d) of HSD-AAM were studied through laboratory experiments, in order to improve the early strength of HSD-AAM without affecting its fluidity; Furthermore, thermogravimetric analysis and differential scanning calorimetry (TG-DSC), X-ray diffraction (XRD), and scanning electronic microscopy (SEM) were used to examine the hydration products and microstructure of the various varieties of HSD-AAM, the mechanism of enhancing the early strength of HSD-AAM by NS content was revealed. The results show that with the increase of NS content, the fluidity and setting time of HSD-AAM decrease, while the early strength increase significantly. When NS content is between 0 and 3%, the fluidity of HSD-AAM decreases by an average of 6.0% with each increase of 1%; the initial and final setting times shorten by 13.7% and 10.0% respectively; 1 d flexural strength and compressive strength increase by 10.7% and 20.2% on average; 3 d flexural strength and compressive strength increase by 23.3% and 12.4% on average; 7 d flexural strength and compressive strength increase by 11.1% and 32.9% on average. This is due to the high pozzolash activity of NS and the hydration reaction with Ca(OH)₂ in the slurry in the early stage, which promotes the hydration process and generates a large amount of C-S-H gel, which improves the loose structure of the slurry and makes the microstructure of the slurry more dense in the early stage.