Effect of Quinolinone·Cymoxanil 40% SC against cucumber downy mildew（Pseudoperonospora cubensis） in cucumber field was studied.Results showed that Quinolinone·Cymoxanil 40% SC was effective against cucumber downy mildew.When 450 g,675 g and 900 g/hm² were applied,the control effects at 14 days after the third application were 65.78%,70.83% and 82.99%,respectively.It was the best when the dosage was 900 g/hm².