In this paper, the extract from Rheum palmatum was tested for prevention effect on powdery mildew of cucumber,stability,duration of the efficacy and restraint spectrums of fungi.The results demonstrated that 100-fold Rheum palmatum extraction was effective to control the disease and its efficacy was superior to 15% WP Triadimefon 1000-fold,which was up to 90.14%. The efficacy was stable in different temperatures and pH values.The duration of the preventive efficacy of Rheum palmatum extraction to S.fuliginea could last over 6 days,and also significantly to restrain many other fungi such as Cochliobolus sativus,Exserohilum turcicum ,et al.