Greenhouse pot experiments were carried out to study the main physiological responses of heat-tolerant variety “Nagina 22” (N22) under high temperature (HT) stress during flowering period, indoor growth chamber was used for the HT conditions for all tested varieties. The results show that: under HT stress, the flowering stage of N22 did not change but the daily flowering amount of spikelet manifested the “peak-lag”; anther dehiscence blocked and germinated pollen on the stigma reduced with a significant level (P<0.05) in heat sensitive variety “Moroberekan” compared with the N22; there was a significant relationship between the number of germinated pollen on the stigma and anther dehiscence, and then showed a strong relationship between seed-setting and the number of germinated pollen on the stigma; main tiller seed-setting was decreasing under HT, and the decline-rate significantly speeded up in “Moroberekan” compared with N22, indicating the true tolerance of N22 and the “cumulative effect” of HT stress on seed-setting, moreover, HT stress effect occurred before pollination.