Abstract: In order to determine the best application conditions of fenpyrazone in corn fields, the effects of temperature, light intensity, interval time of rainfall after herbicide application and weed leaf stage on the herbicidal activity of fenpyrazone against barnyard grass Echinochloa crus-galli were studied using whole-plant dose-response experiments in greenhouse. At the temperature of 15-35 ℃, the efficacy of fenpyrazone was increased with the increase in temperature. At the temperature of 20-35 ℃, the efficacy of fenpyrazone tended to be stable, and the GR₅₀ value for barnyard grass was between 2.8-7.6 g/hm². With the increase in light intensity, the inhibition rate of the fresh weight of barnyard grass by fenpyrazone increased significantly from 41.19% to 90.84%. Fenpyrazone showed strong tolerance to rain erosion and could quickly penetrate the leaf tissues. And there was no significant correlation between its herbicidal activity and the interval time when the rainfall interval after treatment was more than 1 h, with the GR₅₀ value of 1.7-2.4 g/hm² for barnyard grass. The weed leaf stage significantly affected the efficacy of fenpyrazone against barnyard grass. The barnyard grass at 1- to 4-leaf stage was the most sensitive, with the GR₅₀ value of <1.0 g/hm². In conclusion, to maximize the efficacy of fenpyrazone, it should be applied when the weather is sunny, the temperature is higher than 20 ℃ and the targeted weed of barnyard grass is at the 1- to 4-leaf stage. The herbicide needs to be reapplied in case of rainy weather within 1 h after the application. The results provide a scientific basis for further promoting and applying fenpyrazone in corn fields.