The invention discloses an automatic control medical radioactive wastewater attenuation discharge device, system and method. The device comprises a water collecting tank and at least one attenuation tank communicated with a wastewater outlet of the water collecting tank, wherein the attenuation tank comprises a septic tank and at least two U-shaped units which are sequentially connected in series; a water inlet of the septic tank is communicated with a water inlet pipeline; an overflow port in the top part of the septic tank is communicated with a water inlet of each U-shaped unit; each U-shaped unit comprises a left tank, a right tank, and a separating wall for separating the left tank and the right tank; a circulating hole for communicating the left tank and the right tank is formed in the bottom part of the separating wall; the left tank is provided with the water inlet of the corresponding U-shaped unit in the upper side wall of a non-separating wall; the right tank is provided with a top-part overflow port of the corresponding U-shaped unit in the upper side wall of a non-separating wall; a switching valve control loop of each U-shaped unit is controlled centralizedly. According to the automatic control medical radioactive wastewater attenuation discharge device, the system and the method provided by the invention, a modularized independent unit format is adopted, each independent unit collects a half-life period of nuclide according to a principle of a fluid mechanics U-shaped pipe, and a septic tank unit and two to three U-shaped units are made, so that the aims of independent storage, discharge and cleanness are achieved.