Abstract: Greenhouse studies were carried out to determine the suitable time of QYR301 application and rainfastness, examine influence of temperature and light intensity on its efficacy targeted by barnyard grass (Echinochloa crus-galli) and paddy (Oryza sativa). QYR301 showed high safety for paddy at different leaf stages, with 0% dry weight inhibition for over-2 leaf stage of the paddy at 360 g a.i./hm². Meanwhile, the growth stage of barnyard grass significantly influenced the efficacy of QYR301 and barnyard grass at 2-3 leaf stages was most sensitive to QYR301 with 95%-98% dry weight inhibition at 30 g a.i./hm². QYR301 had ability to quickly penetrate into the leaf tissues and remained effective on 8 h of simulate rainfall after application with 75%-93% dry weight inhibition at 30-120 g a.i./hm². QYR301 exhibited relatively stable efficacy against barnyard grass under temperature of 20-35 ℃ with GR₅₀ of 44.6-51.4 g a.i./hm². Compared with middle (45%) and low (4%) light intensity, QYR301 showed significant higher efficacy against barnyard grass under high light intensity (100%) conditions. In conclusion, the growth stage of weeds, rainfall, temperature and light intensity significantly influenced the efficacy of QYR301 and the optimum time, temperature and light intensity for barnyard grass control were 2-4 leaf stage of the weeds and over-2 leaf stage of the paddy, 20-35 ℃ and 100% light intensity, respectively. The application needs to be reconducted if the rain occurs within 8 hours after application.