Abstract: Excess air in heaters of delayed coker unit will not only affect the thermal efficiency, but also affect the operating period of the heater. The coke deposition mechanisms of the heater tubes were briefly introduced, and the factors leading to tube coking were analyzed. Using the professional engineering design software, the influences of excess air on the heat duty of convective and radiant section of coker heater, the furnace tube surface heat intensity, the tube wall temperature and the flame temperature were discussed respectively. The influence on the operating period of the heater was analyzed simultaneously. When the excess air in the heater of a delayed coker unit was increased from 5% to 13%, the combustion chamber temperature might decrease from 715.8 ℃ to 709.4 ℃, the tube wall temperature might decrease from 510.3 ℃ to 505.8 ℃. The results showed that higher excess air could reduce the surface heat intensity of radiant tube, thus lowered the radiant tube surface temperature and tube coke deposition tendency, and extended the operating period of the heater as a result.