The present invention belongs to the field of pesticides, particularly relates to a pyrazolone compound or a salt thereof, a preparation method therefor, a herbicidal composition and use thereof. The pyrazolone compound is as described in formula I:In the formula, R1R2N represents substituted or unsubstituted 3-8 membered nitrogen-containing heterocyclic group containing 1-3 heteroatoms； or R1 and R2 each represent hydrogen or C1-8 alkyl； R3 represents hydrogen, C1-4 alkyl, alkenyl, alkynyl, unsubstituted C3-6 cycloalkyl or C3-6 cycloalkyl substituted by C1-4 alkyl； R4 represents methyl, ethyl, n-propyl, isopropyl or cyclopropyl； X represents hydrogen, -S(O)nR6, -R7 or substituted or unsubstituted 3-8 membered heterocyclic group containing 1-4 heteroatoms, wherein, n represents 1, 2 or 3, R6 represents substituted or unsubstituted alkyl or aryl, and R7 represents substituted or unsubstituted alkyl, aryl, alkyl acyl or aroyl. The active substance of the invention exhibits good herbicidal effect, and is convenient to use, low in cost and has great commercial value.
