Conventional silica-enriched well cement often experiences significant strength retrogression during long-term exposure to high-temperature deep-well conditions. To address this issue, the development of high-temperature-resistant cement systems is essential for ensuring long-term zonal isolation in ultra-deep wells. This study investigates the high-temperature performance evolution of traditional silica-enriched cement and metakaolin-modified silica-enriched cement. Results show that the metakaolin-modified system exhibits superior mechanical strength, microstructural integrity, and hydration phase composition after 90 days of curing. Furthermore, metakaolin effectively mitigates strength retrogression at different silica sand contents (40% and 70%), indicating its strong potential for enhancing the thermal stability of well cement under high-temperature conditions.