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, as well as the underlying mechanisms, and the criteria for high-temperature stress in early rice remain unclear. In this experiment, the primary rice variety ‘Xiangzaoxian 45’ was used to investigate these aspects. Rice plants with pots during the grain-filling stage were subjected to different high-temperature treatments in artificial climate chambers. The experiment was conducted in two phases, each with six temperature treatments ranging from 35 ℃ to 42 ℃ and lasting for four different durations: 3, 5, 7, and 10 days. The first phase (35 ℃, 38 ℃, 40 ℃) ran from June 22 to July 1, 2023, and the second phase (37 ℃, 39 ℃, 42 ℃) from July 2 to July 11, 2023. Each temperature treatment was applied from 12:00 to 17:00 daily, with natural sunlight and a controlled relative humidity of 80%. After the treatment period, 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, while rice plants in the field during the same period served as the control group. We investigated the effects of high temperatures on photosynthetic parameters, chlorophyll content, yield components, and grain quality of early rice, as well as the mechanism and grading index of high-temperature stress. According to meteorological and disaster data from typical years and planting counties, the accuracy rate of the grading index was verified. The results indicated that under high temperatures, compared to the control group, there was a decrease in chlorophyll content, net photosynthetic rate, transpiration rate, and stomatal conductance of flag leaves, as well as a reduction in 1000-grain weight and yield, and an increase in the blighted-grain rate. These variations were proportional to the temperature intensity and duration. Furthermore, high-temperature treatments led to a decrease in the milled rice rate, head milled rice rate, and amylose content, but an increase in the chalky grain rate, chalkiness, and protein content. The starch viscosity characteristic (RVA profile ) parameters of early rice were also altered under different high-temperature conditions. We established a grading index for high-temperature stress in early rice: daily maximum temperature (T_max) < 35 ℃ indicates no high-temperature stress; 35 ℃ ≤ T_max < 37 ℃, and the number of consecutive days (D_n) < 3 days indicates no stress; 35 ℃ ≤ T_max < 37 ℃ (D_n ≥ 3 days) or 37 ℃ ≤ T_max < 38 ℃ (3 days ≤ D_n < 6 days) is considered mild stress, corresponding to 0 < accumulated hot damage temperature (H_a) ≤ 11, 0% < decrease amplitude of rice yield (Y_d) ≤ 10%; 37 ℃ ≤ T_max < 38 ℃ (D_n ≥6 days) or T_max ≥ 38 ℃ (3 days ≤ D_n < 6 days) is classified as moderate stress, corresponding to 11 < H_a ≤ 22, 10% < Y_d ≤ 20%; T_max ≥ 38 ℃ (D_n ≥ 6 days) is severe stress, corresponding to H_a > 22, Y_d > 20%. Verification using data from typical years and counties showed an index accuracy rate of 85.45%. This suggests that daily maximum temperatures of 35 ℃ 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 based on daily maximum temperature and the number of consecutive days of stress. The index categorizes stress into no stress, mild stress, moderate stress, and severe stress, with corresponding ranges for accumulated hot damage temperature and decrease amplitude of rice yield. According to the early rice disaster index, it is possible to predict the level of high-temperature stress. The findings provide a scientific basis for monitoring and evaluating high-temperature stress in early rice, which can aid in agricultural disaster reduction and improving agricultural efficiency.