The present invention provides a method for preparing nintedanib. The method of the present invention is carried out by using 4-halo-3-nitro-benzoic acid methyl ester (compound II) and 3-oxo-3-phenylpropionate (compound III) as raw materials, and preparing nintedanib via intermediates of 4-(1-alkoxy-1,3-dioxo-3-phenylpropan-2-yl)-3-nitrobenzoic acid methyl ester (compound IV) and 3-benzoyl-2-oxoindoline-6-methyl formate (compound V). The method for preparing nintedanib (I) provided by the invention has the advantages of using easily obtained raw materials, having a simple process, being cost effective and environmentally friendly, and is suitable for industrial-scale production.