In order to explore the mechanism by which cotton resists feeding by aphids, a spectrophotometer was used to measure physiological indices of cotton under attack by different densities of aphids. During the flowerbolling stage, although chlorophyll content first increased, the chlorophyll content of plants subject to attack by 80 aphids was significantly lower than that of control plants after 10 days. Soluble protein content increased under all aphid densities with duration of feeding time; that of plants subject to attack by 80 aphids was 7838% higher than that of control plants after 10 days. Free proline and MDA content first increased, then decreased, with feeding time and gradually increased with aphid density. In conclusion, cotton can withstand attack by a low density of aphids, but is sensitive to relatively high aphid densities. The plants’ defensive ability increases with time of exposure to aphid attack.