Abstract: Salicylic acid (SA), acetylsalicilic acid (ASA), and 14-hydroxylated brassinosteroid (HBR) are natural, endogenous plant growth regulators with high efficacy. Spraying those exogenous substances can effectively alleviate the adverse effects of heat stress in rice, but its physiological mechanisms, and the quantitative effects on increasing rice yield are still unclear. In this study, the dominant rice variety in Jiangxi Province ‘Jinliangyouhuazhan’ was used to investigate these aspects. A 2-year (2020, and 2021) experiment was conducted involving four sowing dates (April 25, May 5, May 15, and May 25) to investigate the effects of SA, ASA, HBR, and the sowing date on the physiological characteristics, and yield of rice subjected to high temperature stress. Under ambient high temperature conditions (daily average temperature ≥ 30 ℃ for ≥ 3 days or maximum temperature ≥ 35 °C for ≥ 3 days), 0.50 mmol·L⁻¹ SA, 0.50 mmol·L⁻¹ ASA, 0.20 μmol·L⁻¹ HBR), and their combinations (SA+HBR, ASA+HBR) were applied to leaves from the late booting stage to the flowering stage, with clean water as a control (CK). Compared with the CK treatment, rice plants treated with SA, ASA, HBR, SA+HBR, and ASA+HBR had a higher soluble sugar content, soluble protein content, proline content, Relative chlorophyll content (SPAD value), superoxide dismutase enzyme activity, peroxidase enzyme activity, and catalase enzyme activity but a lower malonaldehyde content in flag leaves. SA, ASA, HBR, SA+HBR, and ASA+HBR increased the number of differentiated spikelets per panicle, but decreased the degenerated spikelet percentage under high temperature. Compared with the CK treatment, the SA, ASA, HBR, SA+HBR, and ASA+HBR treatments increased the number of grains per panicle, seed-set rate, and yield under high temperature conditions. Moreover, the number of grains per panicle and actual yield increased on average by 5.01% and 5.69%, with the SA treatment; by 4.40% and 4.00%, with the ASA treatment; by 5.94% and 7.95%, with the HBR treatment; by 8.15% and 12.30%, with the SA+HBR treatment; and by 6.73% and 10.3%, with the ASA+HBR treatment, respectively. Interestingly, the ranking of the five treatments was as follows: SA+HBR > ASA+HBR > HBR > SA > ASA > CK. Compared with those sown on April 25 and May 5, plants sown on May 15 and May 25 had suitable temperature conditions, a higher accumulated temperature with average daily temperature ≥ 10 ℃ (by 2.07%), fewer days during which average temperature was ≥ 30 ℃ (average decrease of 4.00 days), and fewer days during which the maximum temperature was ≥ 35 ℃ (average decrease of 1.38 days) from the late-booting stage to the he mature stage. The number of grains per panicle and grain yield under the last two sowing dates (May 15 and May 25) was higher than that under the first two sowing dates (April 25 and May 5), with average increases of 9.13% and 7.98%. Therefore, the application of SA, ASA, HBR, SA+HBR, ASA+HBR and the use of an optimal sowing date (from May 15 to May 25) increased the single-season rice yield under high temperature condition.