Abstract: The problems of low temperature and shallow water-gas flow are concerned in deepwater cementing.The properties of the composite deepwater cementing system combining sulfoaluminate cement with Class-G oil well cement were tested,including the static gel strength,thickening time,compressive strength,long-term compressive strength and volume expansion of cement stone.The hydration products and the long-term products under different temperature were characterized by X-ray diffraction.The results demonstrate that the ettringite(AFt) produced in the early hydration of sulfoaluminate cement can enable the cement slurry to have the favorable behaviors of low temperature and early strength,right-angle-set property and short transition time of static gel strength,as well as the cement stone showing negligible volume expansion.During the latter hydration period,the calcium silicate gel(C₂SH₂) and Ca(OH)₂ are created from the hydration of C₃S and C₂S.C₂SH₂ is closely packed in the AFt crystals to compact the structure of cement stone and promotes strength continuously.The main stable hydration products of AFt and C₂SH₂ can guarantee the long-term mechanical stability of cement stone.