The invention provides a preparation method of trimethylaluminum. The preparation method comprises the following steps: SI, in the presence of a bismuth-based compound and a solvent, carrying out a reaction on trialkylaluminium and halogenated methane to obtain a reaction product; and SII, removing halogenated alkane in the reaction product, then leaving the reaction product to stand to separate out crystals, recovering the crystals separated out and separating the residual reaction product to obtain trimethylaluminum. The preparation method adopts a high pressure liquid phase reaction, raw materials are in full contact, the reaction efficiency is high and the conversion ratio of trimethylaluminum is high. A recovery process of a catalyst is simple and the recovery rate is high, so that the production cost is lowered obviously.