The present invention relates to a sewage/wastewater treatment system comprising a rectangular upflow anaerobic/oxygen-free reaction tank and a method of treating sewage/wastewater using the same. The rectangular upflow anaerobic/oxygen-free reaction tank comprises a water introducing unit, an inlet unit being in fluid communication with the water introducing unit and having a plurality of through-holes formed therein through which water introduced from the water introducing unit can pass, a mixing unit comprising a main shaft and a plurality of spaced-apart stirring blades fixed to the main shaft, and an outlet unit for collecting water after being treated in the anaerobic/oxygen-free reaction tank. The system and method can treat sewage/wastewater cost-effectively.