Abstract: To demonstrate the sensitivity of Chinese cabbage downy mildew pathogen Peronospora parasitica to fluopicolide, concave-slide moisturizing method and cotyledon in vitro soaking method were used to test the inhibiting effect of fluopicolide on different development stages of P.parasitica. The results showed that fluopicolide had inhibitory activity on sporangium germination, germ tube elongation, lesion expansion, sporangiophore emergence and sporangium production of P.parasitica to different extents, among which the inhibition of fluopicolide on sporangium germination and germ tube elongation was the most significant, with EC50 values of 0.113 3 and 0.291 8 μg/mL, respectively. The inhibition rates of fluopicolide at 8.00 μg/mL on lesion expansion and sporangium production of P.parasitica were 86.08% and 72.48%, respectively, and the inhibition on sporangiophore emergence was also significant. At the same time, cotyledon in vitro soaking method was used to test the toxicity of fluopicolide to 60 isolates of P.parasitica collected and isolated from different areas of Shandong province. The results indicated that the difference in sensitivity of the 60 isolates to the fungicide was small, the sensitivity frequency showed a nearly Gaussian distribution, and the EC50 values were in the range of 0.163 2-0.802 2 μg/mL, suggesting the EC50 mean value of 0.405 3 μg/mL could be used as the sensitivity baseline of P.parasitica to fluopicolide.