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When cementing geothermal wells in dry hot rocks, the bottom temperature is often as high as above 200 ℃. Aiming at the problem of strength decline of cement paste caused by high temperature at the bottom of the well under dry-hot rock conditions, this paper directly starts from the chemical composition of cement, and improves this problem by adjusting the ratio of C3S and C2S, and compounding low-heat portland cement with lower calcium-silicon ratio under the synergistic effect of silicon powder. Firstly, the mechanical properties of multiphase C3S-C2S mineral system are the best when C3S∶C2S=1.0. Combined with XRD, TGA and SEM tests, it is known that the reduction of calcium-silicon ratio has a positive effect on the formation of favorable phase xonotlite. Lowgrade cement with lower calcium-silicon ratio is introduced to strengthen G-grade cement. The results show that the compressive strength of "30% G-grade +70% low-grade cement" composite cement system (the ratio of C3S to C2S is 1.07) is 27.34 MPa under the action of 40% silicon powder. The high temperature resistance of cement paste can be greatly improved from the cement itself by properly adjusting the mineral composition in cement to make the ratio of C3S to C2S about 1.0. [ABSTRACT FROM AUTHOR]