The invention provides a method for hydrogenating residual oil. The method comprises the steps of firstly carrying out hydrogenating protection reaction on the raw material residual oil in a hydrogenating protection device so as to obtain hydrogenating protection products, then, carrying out hydrotreating reaction on the hydrogenating protection products in a hydrotreater so as to obtain hydrotreating products, and finally, carrying out separation on the hydrotreating products in a separating unit, thereby obtaining recycle hydrogen and a product. The method is characterized in that the hydrogenating protection products are divided into gas-phase and liquid-phase products, the gas-phase product is directly mixed with the hydrotreating products, then, separation is carried out, part of the liquid-phase product is mixed with a mixed hydrogen raw material, the hydrogenating protection reaction is carried out, and the other part of the liquid-phase product is fed to a hydrotreating reactor for hydrotreating reaction. Compared with the prior art, the method provided by the invention has the advantages that the lives of a hydrotreating catalyst and a hydrogenating protection catalyst can be prolonged, and the selectivity of a target product of the hydrogenating protection reaction is improved.