NOVELTY - Supercritical treatment of a dehydrated sludge in a sewage plant comprises placing a dewatered sludge in a sludge storage tank, and placing the sludge in a heat exchanger at 250 degrees C; preheating after pressurizing the sludge to enter a high pressure fluidized bed reactor at 400-500 and 25-30 MPa; feeding gas-liquid mixed phase in the heat exchanger at 250 degrees C, and conveying dipotassium and magnesium chloride to magnesium ammonium phosphate crystal and discharging system; uniformly mixing the reaction tank material; directly discharging supernatant; and discharging solid phase residue.    USE - Method for the supercritical treatment of a dehydrated sludge in a sewage plant (claimed).    ADVANTAGE - The method is inexpensive. The heat of the ammonia nitrogen in liquid phase product is recycled, thus reducing secondary pollution.    DETAILED DESCRIPTION - Supercritical treatment of a dehydrated sludge in a sewage plant comprises:    (A) placing a dewatered sludge in a sludge storage tank, and placing the sludge in a heat exchanger at 250 degrees C;    (B) preheating after pressurizing the sludge to enter a high pressure fluidized bed reactor at 400-500 degrees C and 25-30 MPa, when the dewatered sludge in the reactor reaches supercritical water state, and stopping for gasification reaction for 10 minutes;    (C) feeding gas-liquid mixed phase in the heat exchanger at 250 degrees C, reducing the gas-liquid mixing to normal temperature and normal pressure, feeding the gas phase product in a gas pipe network, feeding the liquid phase product through a reaction tank, respectively opening dipotassium and magnesium chloride metering pump, and conveying dipotassium and magnesium chloride to magnesium ammonium phosphate crystal, and regular discharging system;    (D) starting a low-speed motor for uniformly mixing the reaction tank material;    (E) directly discharging supernatant, or circulating water containing a large amount of ammonia nitrogen, and recovering ammonia nitrogen that can be used as nitrogen fertilizer; and    (F) discharging solid phase residue according to the content of heavy metal sludge, processing, if low heavy metal content, the solid phase residue can be used as fertilizer, and if the content of heavy metal is high, the solid phase residue directly buried, or can be used as a wastewater handling material.