The invention discloses a system and method for constant control of effluent. The system for constant control of the effluent is directly arranged on a water treatment system so as to perform constantcontrol on the effluent of the water treatment system, and the method for constant control of the effluent includes the step of regulating the speed of the effluent continuously in a reservoir of thewater treatment system through a PLC so as to ensure that the effluent speed is always constant. The system has a simple structure and convenient installation and use, the productivity is improved, and large cost of human resources is saved, so that stability of the water flow of the water treatment system is realized, waste of excessive chemical addition caused by reduction of the flow is avoided, influences of human factors on the treatment amount of water can be avoided or reduced, and energy conservation and consumption reduction are achieved to a maximum extent.