The invention relates to a method and a device for secondarily filtering a doctor solution by side stream, and belongs to the technical field of purification of the doctor solution of synthesis ammonia. The method adopts a doctor solution side-stream filter process, and desulfurizes the doctor solution by a doctor solution side-stream filter system and a filter regeneration system which regenerates the filter system; and the device consists of the doctor solution side-stream filter system and the filter regeneration system. The invention is suitable for tannin extract normal pressure desulfurization systems; and after the doctor solution shunted on a doctor solution main pipeline is desulfurized by the device, suspending sulfur and secondary salt contents in deficient liquid after the desulfurization can be effectively reduced, and phenomenon of tower blockage of a desulfurizing tower is reduced. The method and the device not only improve the desulfurization efficiency of the tannin extract desulfurization systems and prolong the stable operation cycle of the systems, but also reduce the discharge of the doctor solution due to tower cleaning; and the suspending sulfur particles after the desulfurization are not discharged and have no pollution, the phenomenon of the tower blockage of the desulfurizing tower is reduced, and the system is stably operated in a long cycle; and themethod and the device realize online operation and online back washing, and optimally configure a back washing solution to prevent the back washing solution polluting the environment.