The invention discloses a biological method sewage purification system, and belongs to the technical field of sewage treatment. The biological method sewage purification system comprises a water distribution channel, a water outlet channel, a purification tank and a fishing device, a water inlet hole and a PH regulating tank are arranged at the front end of the water distribution channel; the rearend of the water distribution channel is the sewage purification tank; water inlets are uniformly formed in the wall of the water distribution channel; artificial aquatic plants are uniformly fixed at the bottom of the sewage purification tank, a moss fishing device is arranged at the top of the sewage purification tank, moss is conveyed to a moss stacking pond beside the sewage purification tankthrough a conveying crawler belt, a water temperature adjusting device, a blowing type aerator and an LED lamp strip are arranged in the sewage purification tank, the sewage purification tank is connected with a water outlet channel through a rotatable elbow, and the water outlet channel is connected with a water outlet pipe. By adjusting the PH value, temperature, dissolved oxygen and illumination of a water body, the moss is induced to grow rapidly, salvage and outward transportation are conducted in time, pollutants contained in the sewage are consumed continuously, and therefore the effects of sewage treatment and water quality purification are achieved.