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 g
