Abstract: High temperatures are detrimental to the growth and development of early rice, particularly during the grain-filling stage. However, the specific responses to varying high temperatures and the related underlying mechanisms as well as the criteria for high-temperature stress in early rice remain unclear. In this study, the dominant rice variety in Hunan Province ‘Xiangzaoxian 45’ was used to investigate these aspects. Potted rice plants at the grain-filling stage were subjected to different high-temperature treatments in artificial climate chambers. The experiment was conducted in two phases, each with three temperature treatments ranging from 35 °C to 42 °C and lasting for four different durations: 3, 5, 7 and 10 days. The first phase (35 °C, 38 °C and 40 °C) lasted from June 22 to July 1, 2023, while the second phase (37 °C, 39 °C and 42 °C) lasted from July 2 to July 11, 2023. Each temperature treatment was applied daily from 12:00 to 17:00 under natural sunlight and controlled relative humidity of 80%. After treatment, the climate chambers were ventilated to ensure air circulation. At the end of each treatment cycle, the rice plants were returned to the field to grow naturally. Rice plants grown in the field during the same period served as the control group. The effects of high temperatures on the photosynthetic parameters, chlorophyll content, yield components, and grain quality of early rice were investigated, and the mechanism and grading index of high-temperature stress were anaylsed. The accuracy of the grading index was verified using meteorological and disaster data from typical years and planting counties. High temperatures resulted in decreases in the chlorophyll content, net photosynthetic rate, transpiration rate, and stomatal conductance of flag leaves as well as 1000-grain weight and yield, and an increase in the blighted-grain rate compared with the respective results of the control group. These variations were proportional to the temperature intensity and duration. Furthermore, high-temperature treatments led to decreases in the milled rice and head milled rice rates and amylose content but an increase in the chalky grain rate, chalkiness and protein content. The starch viscosity characteristics (Rapid Visco Analyser profile) of early rice were also altered under different high-temperature theatments. The following grading index for high-temperature stress in early rice was established: a daily maximum temperature (T_max) < 35 ℃ indicates no high-temperature stress; 35 ℃ ≤ T_max < 37 ℃ and the number of consecutive days (D_n) < 3 d indicate no stress; 35 ℃ ≤ T_max < 37 ℃ with D_n ≥ 3 d or 37 ℃ ≤ T_max < 38 ℃ with 3 d ≤ D_n < 6 d represents mild stress, corresponding to 0 ℃·d < accumulated hot damaging temperature (H_a) ≤ 11 ℃·d and 0% < decrease amplitude of rice yield (Y_d) ≤ 10%; 37 ℃ ≤ T_max < 38 ℃ with D_n ≥ 6 d or T_max ≥ 38 ℃ with 3 d ≤ D_n < 6 d represents moderate stress, corresponding to 11 ℃·d < H_a ≤ 22 ℃·d and 10% < Y_d ≤ 20%; and T_max ≥ 38 ℃ with D_n ≥ 6 d represents severe stress, corresponding to H_a > 22 ℃·d and Y_d > 20%. Verification using data from typical years and counties revealed an index accuracy of 85.45%. This suggests that a daily maximum temperature of 35 °C or higher for three consecutive days can inhibit photosynthetic performance, affect yield and quality formation, and cause high-temperature stress. We established a grading index for high-temperature stress in early rice on the basis of the daily maximum temperature and number of consecutive days of stress. This index divides stress into none, mild, moderate, and severe categories, with corresponding ranges for accumulated hot damaging temperature and decreased amplitude of rice yield. With this disaster index for early rice, predicting the level of high-temperature stress would be possible. These findings provide a scientific basis for monitoring and evaluating high-temperature stress in early rice, which could aid in reducing agricultural disasters and improving agricultural efficiency.