The invention discloses a treating agent for a drilling fluid and a preparation method thereof. The treating agent is prepared by a reaction of polyether amine and o-dihydroxybenzene through the following steps: fully dissolving the polyether amine and the o-dihydroxybenzene under the condition of reduction, adding a catalyst, maintaining a reaction temperature in a range of 40 to 100 DEG C, and carrying out a reaction for 4 to 12 h so as to obtain an intermediate product; adding a reducing agent, and continuing a reaction for 8 to 24 h so as to obtain a suspension; and removing an organic solvent in the suspension, then placing the solvent-removed suspension into a dialysis bag, carrying out dialysis with deionized water, and carrying out vacuum freeze-drying so as to obtain the treatingagent. The treating agent provided by the invention has good compatibility with a frequently-used water-based drilling fluid; and the drilling fluid treated by using the treating agent provided by theinvention has the advantages of prominent wall-bracing performance, long action time, no pollution, low cost, etc.