The invention provides a synthesis method of trimethoxyboroxine. Borane methyl sulfide serves as a raw material to react with carbon dioxide, trimethoxyboroxine is obtained, borane methyl sulfide is added into a Schlenk reaction bottle at the temperature of -60 - -80 DEG C, THF (tetrahydrofuran) is added into the Schlenk reaction bottle to be mixed for 15-20 seconds under the pressure of 0.5-0.7Pa, carbon dioxide is guided into the reaction bottle, the pressure is controlled to be 90-100kPa, then heating is conducted till the temperature reaches 70-80 DEG C, it is detected that no borane methyl sulfide exists, the reaction is ended, and trimethoxyboroxine is obtained. The preparation method is simple, the preparation technology is clean and environmentally friendly, the preparation processis moderate and stable, the yield of the prepared trimethoxyboroxine is high, the purity is high, the water content is low, and the acid value is low.