The invention discloses a rubber composition for run-flat tire sidewall part of a car. The rubber composition is characterized by consisting of the following raw material components in parts by weight: 40.0-60.0 parts of natural rubber, 40.0-60.0 parts of butadiene rubber, 50.0-80.0 parts of carbon black, 1.0-5.0 parts of a softening agent, 1.0-5.0 parts of tackifying resin, 1.0-3.0 parts of an anti-aging agent, 2.0-5.0 parts of zinc oxide, 1.0-3.0 parts of stearic acid, 2.0-5.0 parts of sulfur, 2.0-4.0 parts of an accelerant and 1.0-3.0 parts of an anti-reversion agent. The formula of the supporting adhesive takes the requirement on three performances into consideration, namely high hardness, low themogenesis and good flexibility, so that the supporting adhesive of which hardness is 75-80, 100% stretching modulus is 6-9MPa and resilience is 58-68 is obtained; therefore, the safety and the availability (travelability) of the tire under a run-flat state are guaranteed, and the driving safety of the car under the run-flat state of the tire is greatly improved.