The invention which discloses alkyl modified 1,3,4-thiadiazoline derivatives, a preparation method thereof and an application thereof belongs to the technical field of the agricultural chemistry. The structural formula of the derivatives is represented by formula I, and the preparation method of the derivatives comprises the following steps: 1, a thiosemicarbazone compound is obtained by carrying out a condensation reaction on a compound of formula II and N-substituted thiosemicarbazide; and 2, compounds of the formula I are obtained through carrying out a ring closure reaction by MnO2. The derivatives and preparations therefrom have good growth inhibition effects on Sclerotinia sclerotiorum, Eggplant Phytophthora germs, Botrytis cirerea, seedbed rhizoctonia solani germs, Pyricularia grisea and Cotton Fusarium Wilt.