In order to investigate the mechanism of flag leaves responded to high temperature, and to provide suggestions for early rice copping with climate warming, the super hybrid rice Tianyou 998 were subjected to high temperature in heading and milk stage respectively (daily highest temperature 32℃, 35℃, 38℃, 40℃, and 42℃ with 6℃ differences a day for 5 days, natural condition as CK) in climate chambers with pot-cultured. The results showed that the physiological and biochemical indexes of flag leaves were changed after heating treatment. Both superoxide dismutase (SOD) and peroxidase (POD) enzyme activities decreased. However, the content of soluble protein, proline, malondialdehyde (MDA) and the relative electrolyte leakage increased as the daily highest temperature increased. The yield components, such as the seed setting rate and weight of 1000-grains declined. The milled rice percentage and head rice percentage was also seriously impacted. On the contrary, both chalky grain percentage and chalkiness degree increased. The physiological and biochemical indexes of flag leaves correlated with yield and quality significantly, except the content of soluble sugar. Furthermore, the flag leaves could be affected seriously when the daily highest temperature was above 38℃ in heading and milk stage, but the yield and quality decreased when the daily highest temperature reached 35℃.