The invention discloses a built-in instantaneous pressure-reduction type draining device, and relates to the technical field of wastewater treatment. The device is characterized in that a flange is mounted at the upper end of a mixing barrel body, wherein an adjusting shaft is connected through a sidewall of the mixing barrel body; a turbine is mounted at the right end of the adjusting shaft; a retaining bolt is mounted on the turbine; the turbine is engaged with a worm; a locking nut is mounted on the adjusting shaft; a coaxial supporting rod is mounted at the front side of the adjusting shaft through an adjusting gasket and a retaining nut; the lower end of the coaxial supporting rod is connected with a draining barrel body; an adjusting connecting rod which is connected with a valve rod is mounted on the adjusting shaft; the valve rod is connected through a guide sleeve, and the lower end of the valve rod extends out of the draining barrel body; the guide sleeve is mounted on the draining barrel body; a valve plate is arranged at a part where the valve rod is connected with the draining barrel body; the valve plate is mounted on the valve rod through the retaining nut. The device is novel in design, and reasonable in structure; the power of a reflowing pump is reduced; the service life of the device is prolonged; the energy is saved.