3-Chloro-4-(3,4-dichloroisothiazol-5-yl)-5-hydroxy-7-methyl-2H-chromen-2-ones, a kind of coumarin derivatives, were synthesized by β-ketoester formation and cyclization. Target compound 5e was crystallized from methanol for structural identification as monoclinic crystal system, space group C2/c with a = 16.2700(6), b = 7.1801(5), c = 23.4861(10) Å, V = 2742.6(2) ų, Z = 8, D_c = 1.756 g/cm³, F(000) = 1456 and μ = 0.827 mm^-1. 8308 Reflections were collected (6.01≤2θ≤50.05°), of which 2428 were unique (R_int = 0.0432) and used in all calculations. The final R = 0.0408 (I > 2σ(I)) and wR = 0.1056 (reflections). In vitro bioassay indicated that compounds 5d & 5e possessed good activity against Botrytis cinerea, Physalospora piricola, Rhizoctonia solani, and Sclerotinia sclerotiorum with lower EC₅₀ values falling between 0.50 and 4.85 µg/mL than that of positive control osthole with its EC₅₀ values between 7.38 and 74.59 µg/mL. In vivo screening showed that 5e exhibited 98% and 95% efficacy against Pseudoperonospora cubensis (Berk. & Curt.) Rostov. at 100 and 50 µg/mL, respectively. Our studies discovered that the combination of bioactive substructures of isothiazole with coumarin was an effective way to novel fungicide development.