Although the alkali activation of fly ash (FA) has been well investigated, the composite application of multiple chemical activators has not been well explored in the high replacement (50% in mass) low-calcium ultrafine FA blended cement (FABC) paste. This study investigated the mechanical enhancement effect of multiple salt activators (CaCl2, Na2SO4 and CaSO4·2H2O) on the FABC paste, and compared with the most widely used alkaline activator (NaOH and Ca(OH)2). The activation mechanisms were analyzed by non-evaporated water content, pH value, XRD and SEM test. The results have shown that the compound activator has a more obvious strength gain for the FABC paste and is higher than that of the control group and the alkali-activated group, which benefits from the higher non-evaporated water content, a mild and lasting alkaline environment (pH value). In addition, the compound activator can play each role of its components, making FA particles participate in hydration reaction earlier. In terms of the cost and environmental impact, FABC paste activated by composite activators is more economical and cleaner than that activated by single alkali and control paste. This paper provides the guidance for the preparation of FABC paste with low alkalinity, high-benefits and eco-friendly.