C4A3S was synthetized and incorporated with gypsum and calcium hydroxide to prepare calcium sulfoaluminate cement, then the strength change induced by AH3 phase and hydration process was investigated. The synthetized C4A3S was characterized by Rietveld quantitative phase analysis and the hydration production was characterized by XRD and TG DTG. The results indicate that with higher AH3 content and lower ettringite(AFt) content, AH3 phase fills in empty spaces, therefore the hardened paste can obtain higher strength. Gypsum can accelerate the hydration reaction, and with the increase of gypsum dosage, the strength of hardened paste increases first, and then decline.