Abstract: The high-temperature mechanical properties of carbon fiber reinforced polyimide resin matrix MT300/KH420 composites were studied both in macro and micro scale, during which the flexural property variation rule for MT300/KH420 composites of laminates with ply orientation of [0°]₁₄ and[±45°/0°/90°/+45°/0°₂]_s were especially analyzed at temperature ranging from ambient temperature to 500℃. Research shows that MT300/KH420 composite has good mechanical property at high temperature. The flexural strength of [0°]₁₄ laminates exhibit a 51% retention at 420℃, while at 500℃ it changes little. At ambient temperature,[0°]₁₄ laminates exhibit brittle fractures with a rough facture surface and crack growth direction along thickness. As temperature rises, resin tends to have a stronger flowability, leading to a viscoelastic behavior, while failure mainly occurs at the loading nose. At 500℃, thermal exposure has caused the decomposition of resin matrix, leaving separations between the bundles of the oxidated fibers. As for[±45°/0°/90°/+45°/0°₂]_s laminates, the flexural properties show a more stable retention at high temperature. Crack along 45° direction at outer surfaces as well as interlamination are the main failure modes. At 500℃, a severe delamination happens. Compared with matrix-dominated laminates, the flexural properties of fiber-dominated ones are less dependent on temperature.