The invention relates to a synthetic method for a soluble subastituted aniline polymer. The method comprises the following steps: mixing a byproduct-tar in a production process for 2-methyl-6-ethyl aniline and/or 2,6-diethyl aniline with inorganic acid to obtain a solution I; adding an oxidant into water and fully dissolving the oxidant to obtain an oxidant solution; and dropwise adding the oxidant solution into the solution I with strong stirring, soaking a product obtained after reaction in ammonia water, and performing water-washing, suction filtration and vacuum drying to obtain the soluble subastituted aniline polymer. The method has the benefits that the whole preparation process is simple, a special device is not involved, the post-treatment is easy, the environment is not influenced, the industrial enlarged production is easy to realize, and the economic utilization values of byproducts in MEA (monoethanol amine) and DEA (diethanol amine) production are improved.