The invention provides a method for recovering and preparing ammonium sulfate from an acrylonitrile device. An evaporator and a heat exchanger in vacuum direct-flow evaporation are used for evaporating and concentrating ammonium sulfate feed liquid containing acrylonitrile, acetonitrile, hydrocyanic acid and polymer impurities and from the acrylonitrile device, the acrylonitrile, the acetonitrile and the hydrocyanic acid in the feed liquid are evaporated out, the vapor is condensed and directly serves as make-up water of a quench tower of the acrylonitrile device, the polymer is separated and removed, and ammonium sulfate mother liquor is thickened and centrifuged to obtain finish products. The method improves energy utilization rate, saves heating energy, performs removing of light components, separation of the polymer and evaporation simultaneously and is few in operation procedure and short in process, and the direct-flow operation reduces conveying of a pump to the feed liquid and reduces operation cost of the device. The vapor is condensed and directly serves as the make-up water of the quench tower of the acrylonitrile device, so that complex following processing procedures are avoided, resources are saved, and comprehensive utilization of water is achieved.