Abstract: To discover novel fungicide molecules with unique structures and excellent bioactivities, chesulfamide was employed as the lead compound, and a thiadiazine ring was introduced to design and synthesize a series of novel thiadiazine sulfonamide derivatives. The fungicidal activities of the target compounds were subsequently evaluated. The results demonstrated that at a concentration of 50 μg/mL, the target compounds exerted a certain inhibitory effect on five tested plant pathogens. Bioassay results indicated that seven compounds achieved inhibition rates exceeding 90% against Sclerotinia sclerotiorum. Specifically, compound C18 showed a 100.00% inhibition rate with an EC₅₀ value of 0.06 μg/mL, which is superior to chesulfamide (1.12 μg/mL) and comparable to carbendazim and procymidone (0.05 μg/mL). Three compounds displayed inhibition rates over 70% against Botrytis cinerea, among which compound C12 reached an inhibition rate of 79.60% with an EC₅₀ value of 8.16 μg/mL. In vivo control efficacy assay demonstrated that at 200 μg/mL, compounds C7, C18 and C23 exhibited control efficacies over 80% against Sclerotinia sclerotiorum, and compounds C7, C9, C13 and C15 showed control efficacies no less than 80% against Botrytis cinerea. This study provides a reference for the development of thiadiazine sulfonamide fungicides.