Synthesis process of key intermediate of dabigatran etexilate

The invention discloses a synthesis process of a key intermediate of dabigatran etexilate, which comprises the following steps: carrying out catalytic reflux reaction on an intermediate compound I and bromoacetonitrile (compound II) under the action of a catalyst, separating and purifying to obtain an intermediate compound III, and carrying out second-step reaction to synthesize a benzimidazole derivative through visible light catalysis of nitrile in a water phase. Enabling the intermediate compound III and the intermediate compound IV to react under the action of illumination and a catalyst to prepare a benzimidazole derivative compound V. The method has the beneficial effects that the used reagents are cheap and easily available reagents, the production cost is greatly reduced, the used solvent is pure water, the reagent cost is greatly reduced, and the method is simple in process step, convenient to operate, suitable for industrial production, high in selectivity, few in by-product, high in product purity