This study utilized low-angle high-resolution X-ray diffraction (XRD) and conventional X-ray diffraction techniques to systematically investigate the C-S-H interlayer spacing and phase composition of synthetic oil well cement with and without the addition of silica flour at temperatures ranging from 110 to 200 °C. Test results indicate that the basal spacing of C-S-H in high-temperature well cement without silica flour is 10.44 ± 0.13 Å and exhibits minimal variation with changing relative humidity (RH). Furthermore, as the temperature rises, the lower-density major phases of C-S-H and portlandite in the cement without silica flour gradually disappear and are substituted by higher-density phases of α-C2SH, kilchoanite, and reinhardbraunsite. This phase transition results in an increase in the total porosity of well cement, which is a primary contributor to both strength retrogression and deterioration of sealing performance in oil well cement. In contrast, the basal spacing of saturated C-S-H in high-temperature silica-enriched well cement measures 11.30 ± 0.15 Å and retains the characteristic of increasing value with rising RH. However, this variation in C-S-H basal spacing decreases with increasing temperature. The incorporation of silica flour ensures that C-S-H remains the major phase in well cement, even at 200 °C. This addition effectively slows both the phase transition rate and the rate of increase in total porosity of the cement. The metastable phase diagram of high-temperature silica-enriched cement developed in this study, along with the associated C-S-H basal spacing data, provides critical insights for the analysis and prediction of the engineering performance of cement sheaths and wellbore integrity in high-temperature high-pressure wells.