This study explored the potential of ultrafine kaolin as a geopolymer raw material, focusing on the reactivity of calcined kaolin products and its influencing factors. Through a series of analytical techniques, including thickening experiments, particle size analysis, SEM, XRD, and FT-IR, the effects of calcination temperature, time, and Si-Al ratio were evaluated. The results indicated that calcination temperature significantly impacts reactivity, with optimal conditions found between 600 °C and 800 °C, yielding metakaolin with thickening times of 12 to 43 min. Temperatures above 900 °C led to the formation of low-reactivity phases like mullite. The Si-Al ratio also influenced thickening time, with durations varying from 97 min at 0.85 to 43 min at 0.9 and increasing to 157 min at 0.95. Calcination time had the smallest effect, with thickening times ranging from 10 to 25 min over 4 to 8 h. Additionally, calcination altered particle size, initially expanding due to thermal expansion and later shrinking due to lattice distortion above 700 °C. By adjusting these parameters, the reactivity of calcined kaolin could be tailored to suit specific needs. This research provides a theoretical foundation for metakaolin’s application in construction materials and offers novel insights into geopolymer preparation.