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, R 1 R 2 N represents substituted or unsubstituted 3-8 membered nitrogen-containing heterocyclic group containing 1-3 heteroatoms； or R 1 and R 2 each represent hydrogen or C 1-8 alkyl； R 3 represents hydrogen, C 1-4 alkyl, alkenyl, alkynyl, unsubstituted C 3-6 cycloalkyl or C 3-6 cycloalkyl substituted by C 1-4 alkyl； R 4 represents methyl, ethyl, n-propyl, isopropyl or cyclopropyl； X represents hydrogen, -S(O) n R 6 , -R 7 or substituted or unsubstituted 3-8 membered heterocyclic group containing 1-4 heteroatoms, wherein, n represents 1, 2 or 3, R 6 represents substituted or unsubstituted alkyl or aryl, and R 7 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.
