The performance of cementing cement stone is prone to decline in high-temperature and complex environments, and the environment containing acidic gases has a significant negative impact on the sealing ability of the cement sheath. In order to improve the performance of cementing slurry at high temperatures, this paper evaluates the effect of ultrafine diabase powder dosage on the performance of cementing slurry. The microstructure of cement paste containing diabase powder was analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results showed that compared with blank cement slurry, the thickening time of cement slurry containing diabase powder was slightly shortened, and the permeability and water loss were reduced. After 30 days of curing, the mechanical properties of cement paste containing 10% diabase powder were relatively more stable. At high temperatures, the compressive strength increased with the extension of curing time, and the compressive strength increased by 46.54% compared to the blank sample. After 14 days of corrosion in a high-temperature CO2 environment,, the corrosion depth of cement paste containing 10% diabase powder decreased by 51.57% compared to blank cement paste. After adding diabase powder into the cementing slurry, the hydration products of the cement stone were optimized. At high temperatures, the microstructure of the cement stone was dense, ensuring good mechanical properties and strong CO2-corrosion resistance.