The invention discloses a method and equipment for preparing polyphosphoric acid by 
phosphoric acid microwave flash evaporation. The method comprises the following steps of heating by microwaves undera vacuum condition; performing 
flash evaporation on raw liquid of 
phosphoric acid, concentrating and polymerizing, so as to obtain the polyphosphoric acid; condensing 
tail gas of 
flash evaporation, and recycling. The equipment comprises a 
microwave generator, a 
microwave waveguide, a pressure 
transmitter, an overhead tank, a liquid intake valve, an 
infrared temperature sensor, a flash evaporationtank, a reaction tank, a liquid outlet valve, a concentrated liquid collecting tank, a valve, a condensing 
tower, a condensate collecting tank, a collecting tank valve, a 
vacuum pump, and a 
tail gasabsorbing device. The equipment has the advantages that the equipment is simple, the heat efficiency is high, the 
energy consumption is low, the technological process is short, and the treatment speedis quick; compared with the traditional method, the chroma, purity and the like of the produced polyphosphoric acid are better; a continuous, semi-continuous or discontinuous independent manner can be adopted for production, and the production manners are diversified; the problems of easiness in 
corrosion of a heat exchange surface and a forced 
circulating pump, high 
electric consumption, low product purity and the like in the traditional 
evaporation process are solved, and the purpose of high-efficiency preparation of the polyphosphoric acid with high polymerizing degree can be realized.