The invention discloses a diesel oil liquid phase hydrogenation method, which comprises the following steps of: allowing fresh diesel oil feedstock to enter a hydrogen mixing pot for hydrogen mixing, entering a liquid phase hydrogenation reactor for hydrofinishing reaction, allowing a reaction effluent to undergo pressure reduction through a pressure-reducing valve instead of undergoing heat exchange, entering a high-temperature low-pressure flash evaporation pot for flash evaporation, allowing part of the obtained liquid phase to recycle and return to the hydrogen mixing pot, mixing with fresh feedstock, entering the hydrogen mixing pot for hydrogen mixing, mixing the other part of the liquid phase and a gas phase obtained by flash evaporation, and separating to obtain the diesel oil product. After a hydrogenated effluent undergoes flash evaporation, most of hydrogen sulfide is removed by flash evaporation, and simultaneously the light fraction in the refined diesel oil is also separated. On one hand, the content of hydrogen sulfide in the heavy fraction is reduced, cyclic accumulation of hydrogen sulfide is avoided, and it is beneficial to deep desulphurization reaction of diesel oil. On the other hand, liquid phase circulation volume is minimized, and investment and operational energy consumption of a circulating pump are reduced.