The invention discloses a technology process for producing biodiesel and a key preparation device. The technology process mainly comprises the following steps of: carrying out hydrolysis reaction on raw material oil and softened water so as to generate fatty acid; carrying out esterification reaction on the fatty acid separated by a cyclone hydraulic separator and methyl alcohol under a certain condition, thereby preparing coarse fatty acid methyl ester, and separating; and distilling the coarse fatty acid methyl ester by a distilling tower, and separating, thereby obtaining fatty acid methyl ester-biodiesel. The key preparation device for achieving the technology process is a reaction tower which mainly comprises a tower body, a heavy separator, a baffle plate, an overhauling port, a distributor, a gas phase methyl alcohol inlet, a material outlet, a water outlet, a heat conduction oil inlet, a heat conduction oil outlet, a heat conduction oil jacket, a material inlet, a water inlet, a safety valve, a temperature meter, a pressure meter and a gas phase methyl alcohol outlet. The technology process for producing biodiesel and the key preparation device have the characteristics of no need of catalysts, no environmental pollution, low reaction pressure and reaction temperature, less energy consumption, low investment and running costs, wide raw material application range, high yield and the like.