The invention relates to the field of industrial sewage treatment, and in particular to a sewage treatment device and a tunnel wastewater treatment system and a method. The tunnel wastewater treatment system comprises an adjusting tank, a conveying device, a mixer, a sewage treatment device and a clean water tank, wherein the adjusting tank, the conveying device, the mixer, the sewage treatment device and the clean water tank are connected in sequence through pipelines; and the sewage treatment device comprises a flocculation tank, a precipitation tank and a filtering tank. The invention aims to provide the sewage treatment device and the tunnel wastewater treatment system and the method, and the problems that a conventional tunnel wastewater treatment technique is too simple, process parameters are not reasonably designed, the discharged water retention time is short, the reaction speed is slow, the quality of effluent is not stable and up-to-standard emission cannot be ensured, are solved.