A series of novel 2-substituted-6-methyl-4-phenyl-3(2H)-pyridazinone derivatives were synthesized. All target compounds were structurally confirmed by ¹H NMR and elemental analysis. An X-ray crystallographic structure determination was carried out in compound 4e. Herbicidal activities were evaluated through Brassica napus and Echinochloa crusgalli (L.) Beauv tests. Bioassay results showed that most of compounds exhibited better herbicidal activities. Quantitative structure-activity relationship studies showed that when the test grass was rape, their herbicidal activity was mainly related with substituent's sterimol parameter B4 and hydrophobic parameter π; when the test grass was barnyard grass, their herbicidal activity was mainly related with the hydrophobic parameter π of the substituents at the 2-position of pyridazinone ring. Some compounds 4 showed inhibitive activity equal to 5b and 5k, but showed no bleaching activity. The action mode in plants may be different in compounds 4 and 5.