This paper investigated the effects of nano-silica (NS), metakaolin (MK), and silica fume (SF) as supplementary cementitious materials on the compressive strength of mortar with high volume fly ash. Through a comparative analysis, the study established the influence patterns of single and combined additions of various cementitious materials on the compressive strength of the mortar. The microstructure and reaction mechanisms of the specimens were analyzed using methods such as scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermogravimetric analysis (TG). The results indicate that when NS or MK is used alone, the compressive strength of fly ash mortar at 28 days increases and then decreases with the increasing dosage. In contrast, when SF is used alone, the compressive strength of the fly ash mortar increases with the increase in SF content. When NS is combined with MK or SF, both exhibit good synergistic effects, and the strength of mortar with various mix ratios shows a certain improvement compared to the single addition groups at the same curing age. Whether used in combination or alone, NS, MK, and SF all effectively enhance the compressive strength of mortars with high volume fly ash. Among them, the combination of 5% NS and 15% SF (N5S15) achieves the highest compressive strength, exceeding that of the pure cement mortar group at 7 days, 14 days, and 28 days, with a maximum strength of 42.32 MPa at 28 days. Microscopic test results indicate that the increase in strength of the mortar specimens is closely related to the hydration products within the specimens, with higher strength specimens containing a richer array of hydration products such as hydrated calcium silicate and hydrated calcium aluminate.