The invention provides a method for preparing trimethoprim, comprising the following steps: (1) performing a condensation reaction by using 3,4,5-trimethoxybenzaldehyde and aniline propionitrile as raw materials; (2) performing a cyclization reaction among condensation reaction products and guanidine compounds; wherein an inertia solution is added in a reaction system in the step (1), in a reflux reaction water is brought to a water knockout vessel; and in the step (2) the condensation compound and the guanidine compounds are dissolved in ethanol, and are heated and reflowed. The preparation method of the invention is convenient in operation, high in yield low in cost and is easy to controlled in industry.