To investigate the deterioration of compressive property of traditional concrete in a high-temperature environment, uniaxial compression tests were conducted on concrete at various high temperatures. Combined analytical techniques—scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermogravimetric-mass spectrometry (TG-MS)—were used to analyze the degradation mechanism. The experimental results indicate that high temperature has a strong temperature-dependent effect on concrete’s compressive strength. As temperature increases (400 °C, 600 °C, 800 °C), concrete’s compressive strength decreases. These decreases are 27.52%, 56.6%, and 80.76% relative to room temperature, respectively. This phenomenon is attributed to the direct link between concrete’s microstructure and its macroscopic mechanical properties—driven by thermal stresses generated during heating and the decomposition of cement hydration products. Temperatures above 400 °C trigger microcrack formation, and microcracks propagate more rapidly with increasing temperature. At temperatures further increasing to 600 °C, fewer cementitious materials are left decomposable; even stable calcium carbonate starts to decompose. At temperatures of 800 °C or more, decarburization occurs, rendering the concrete microstructure loose and porous. Partial separation of aggregates from the paste causes a near-total loss of compressive strength.