Thaumasite sulfate attack (TSA) is a sulfate attack with thaumasite as the product, and the formation process of thaumasite is closely related to the aluminum source in cement-based materials and sulfate in the external environment. In this paper, aluminum source tricalcium aluminate (C3A), erosion medium Na2SO4 solution and monomineral tricalcium silicate (C3S) were used to prepare the cement paste. XRD, FTIR and SEM/EDS were used to characterize the effects of different molar ratios of SO2-4 to C3A (S/Al ratios) on the formation of thaumasite in monomineral C3S cement paste, so as to reveal the formation mechanism of thaumasite and find the methods to inhibit its formation. The results show that when the S/Al ratio is 3, there is no thaumasite detected in the erosion products after 14 months of attack. When the S/Al ratio is 6 and 9, thaumasite can be detected after 3 months, indicating that the higher external sulfate ion concentration is more beneficial for the formation of thaumasite.