A full continuous synthesis method of metronidazole is provided. An aqueous glyoxal solution, an aqueous acetaldehyde solution and aqueous ammonia are mixed and reacted to produce a 2-methylimidazole-containing reaction mixture, which is mixed with a nitric acid solution and then reacted in the presence of concentrated sulfuric acid to obtain a 2-methyl-5-nitroimidazole-containing reaction mixture. The 2-methyl-5-nitroimidazole-containing reaction mixture is divided by a splitter, such that one part is used to replace concentrated sulfuric acid, and the other part is mixed with formic acid, and undergoes a ring-opening reaction with ethylene oxide to obtain a metronidazole solution. The metronidazole solution is adjusted to pH 2-6 and filtered to obtain a filtrate, which is adjusted to pH 8-14 and filtered to obtain a crude product. The crude product is subjected to decoloring, crystallization, filtration and drying to obtain pure metronidazole with a purity greater than 99.9%.