Abstract: In order to discover novel compounds with high fungicidal activities, a series of novel compounds were designed and synthesized based on the structural modification of 5-(1-(4-methyl-2-oxo-1-oxaspiro4,5dec-3-en-3-yl)ethylidene)-2-aminoimidazolin-4-one derivatives, and introduction of a benzyl group at the 3-position of imidazolinone. Their structures were characterized by ¹H NMR, ¹³C NMR and HR-ESI-MS spectral data. Furthermore, these results of HPLC analysis verified that the Z-configuration of compound 6 would be reversed to the E-configuration of 7 through nitrogen protonation, ring opening, and ring reclosing processes under acidic conditions. The preliminary in vitro evaluation results showed that the introduction of the 3-benzyl group significantly improved the fungicidal activity of this type of compounds, and the EC₅₀ value of 3-benzyl-5-(1-(4-methyl-2-oxo-1-oxaspiro4,5dec-3-en-3-yl)ethylidene)-2-(4-methoxy-phenylamino)-imidazolin-4-one ( 9c ) and 3-benzyl-5-(1-(4-methyl-2-oxo-1-oxaspiro4,5dec-3-en-3-yl) ethylidene)-2-(4-fluorophenylamino)-imidazolin-4-one ( 9h ) against Sclerotinia sclerotiorum were 14.3 and 21.1 mg/L, respectively. In the in vivo evaluation, the efficacies of compounds 9c were 80% and 85% against cucumber downy mildew and wheat powery mildew at a concentration of 400 mg/L, respectively.