The invention discloses a synthesizing method for binimetinib. The method comprises the steps that 2,3,4-trifluoro-5-nitrobenzoic acid and O-(2-tert-butoxy ethyl)hydroxylamine are subjected to a condensation reaction, and obtained N-(2-tert-butoxy ethyoxyl)-2,3,4-trifluoro-5-nitrobenzamide and ammonium hydroxide are subjected to an ammonolysis reaction; obtained N-(2-tert-butoxy ethyoxyl)-2,4-diamino-3-fluoro-5-nitrobenzamide and formic acid are subjected to a ring-closure reaction in a Pearlman's catalyst; obtained N-(2-tert-butoxy ethyoxyl)-6-amino-7-fluoro-3H-benzimidazole-5-formamide and 4-bromo-2-fluoro-1-iodobenzene are subjected to a substitution reaction; obtained N-(2-tert-butoxy ethyoxyl)-6-[(4-bromo-2-fluoro-phenyl)amino]-7-fluoro-3H-benzimidazole-5-formamide is firstly subjected to a methylation reaction with methyl iodide and then subjected to a deprotection reaction with phosphoric acid, and binimetinib is obtained. The synthesizing method is short in route, operation is simplified, cost is low, and the synthesizing method is environmentally friendly and suitable for industrial production.