The invention relates to a deep well sewage aeration technology, an air floatation separation technology and equipment of the technologies, and concretely relates to a horizontal well sewage treatment plant and a method thereof. The horizontal well sewage treatment plant which comprises a total tube of compressed air, branch tubes of the compressed air, an extraneous sewage pipeline, diffuser tubes, a sewage pipeline, exhaust pipelines, a sludge pump, a sedimentation tank, backflow sludge pipelines, a mixed liquid pipeline, and deep wells is formed by an anaerobic zone, an anoxic zone, an aerobic zone, and a backflow apparatus. The method comprises the following steps: leading extraneous sewage to a left-most deep well in the anaerobic zone, carrying out full oxygen charging and mixing on the compressed air and the backflow sludge in the anaerobic zone and the anoxic zone, going through the aerobic zone to the sedimentation tank for solid-liquid separation, settling the sludge to the bottom of the tank with the sedimentation tank, allowing a residual liquid to flow out of an outlet pipeline, and allowing the sludge in the sedimentation tank to back to a fist deep well and to be mixed with the compressed air by going through the backflow sludge pipelines at the bottom of the sludge pump with the sludge pump. The horizontal well sewage treatment plant has the advantages of high oxygen conversion rate, simple structure, high efficiency, and low cost.