In the study of ultra-high temperature cementing systems, it is generally believed that the slurry performance of well cement is primarily determined by chemical additives, while its physico-mechanical properties are influenced by the incorporation of mineral admixtures. This study investigates the effects of different additives on the physico-mechanical properties of well cement systems after curing at 240 °C for various durations. The results show that chemical additives can significantly affect the physico-mechanical properties of cement by altering its setting temperature. Among the three formulations tested, the well cement formulation H70, which includes a complete set of chemical additives such as suspending agents, retarders, dispersants, and fluid loss reducers, and satisfies the high-temperature setting and hardening conditions at 240 °C, achieved a 2-day compressive strength of 44 MPa. However, during the 90-day long-term curing process, the cement exhibited notable pore coarsening, increased porosity, and strength retrogression. In contrast, the cement formulation H70-R, which contains all additives except the retarder, reached a 2-day strength of only 15.5 MPa. Over the 90-day curing period, it showed an increase in strength and a reduction in pore size, but porosity increased. The cement formulation H70-A, which only contained the suspending agent as chemical additive, achieved a 2-day strength of 32 MPa. During the 90-day curing process, its strength continued to improve, with pore refinement and nearly unchanged porosity. Through high-temperature, high-pressure thickening time experiments simulating the heating process in curing vessels, it was found that the setting temperatures for formulations H70, H70-R, and H70-A were 240 °C, 160 °C, and 90 °C, respectively. This difference in setting temperature is the primary factor responsible for the significant variations in the physico-mechanical properties of cement paste in similar systems.