Abstract: To determine the optimal application conditions for the rice field herbicide flusulfinam, this study employed a greenhouse pot experiment using barnyard grass (Echinochloa crus-galli) and rice (Oryza sativa) variety ‘Yongxian 15’ as test targets to investigate the appropriate timing of flusulfinam application, its rainfastness, and the effects of temperature and light intensity on its herbicidal activity. Flusulfinam showed a high level of safety for rice at 0-6 leaf stages, with less than 10% fresh weight inhibition for 1-6 leaf stages of the rice at 360 g/hm² (active ingredients, the same below). Meanwhile, the fresh weight inhibition rate of flusulfinam on barnyard grass was also significantly correlated with its leaf age. Barnyard grass at 1-3 leaf stages was more sensitive to flusulfinam with 96%-98% fresh weight inhibition at 30 g/hm². Flusulfinam demonstrated excellent rainfastness. When applied at doses between 15-60 g/hm², the fresh weight inhibition rates against barnyard grass ranged from 74% to 99%, even after simulated rainfall 48 hours post-application. Flusulfinam exhibited relatively stable efficacy against barnyard grass under temperatures of 20-35 ℃ with GR₅₀ values of 2.0-5.9 g/hm². Compared with middle (45% light, 891 lx) and low (4% light, 79 lx) light intensity, flusulfinam showed significant higher efficacy against barnyard grass under high light intensity (100% light, 1975 lx) conditions. Therefore, the growth stage of weeds, along with rainfall, temperature, and light intensity, significantly affects the efficacy of flusulfinam. To achieve maximum efficacy, it is recommended to apply foliar spray treatments during clear weather with temperatures exceeding 20 ℃, specifically targeting the period after the first leaf stage of rice and the 1-3 leaf stage of barnyard grass. Applications should be made either before 10:00 AM or after 4:00 PM. If rainfall occurs within 48 hours after application, a reapplication should be considered. These findings provide scientific technical support for promoting and applying flusulfinam in rice fields.