The invention discloses a portable and movable slow-flow riverway water body treating device and method. The device comprises a casing with a water inlet and a water outlet, wherein the casing is internally divided into a filtering area communicated with the water inlet, an aerobic zone communicated with the filtering area and positioned at the upper half part of the casing, an anaerobic zone communicated with the aerobic zone and positioned at the lower half part of the casing, and a water outlet area communicated with the anaerobic zone and the water outlet; the device further comprises a biological rotating disc arranged in aerobic zone, a solar-tracking photovoltaic system for driving the biological rotating disc, a filling material that the aerobic zone is filled with, water pumping devices respectively arranged at the water inlet and the water outlet, and a floater device for suspending the casing in the water body. The device floats on the water surface of a polluted riverway, does not occupy limited urban land source, does not generate secondary pollution, adopts the solar-tracking photovoltaic power generation system to supply power, is low in operating cost, and overcomes the defects of the riverway treatment technology.