Method for preparing polyisocyanate and reaction device therefor

Provided are a method for preparing a polyisocyanate, a polyisocyanate obtained by using the method, and a reaction device used in the method. The method comprises the following steps: 1) subjecting a polyamine solution and a phosgene solution to a cold reaction of a phosgenation reaction; and 2) further subjecting the material from step 1) to a hot reaction, wherein the amount of monofunctional impurities produced in step 1) is controlled such that the content of the monofunctional impurities in the final product is 0.1 ppm to 20000 ppm, and the amount of polyfunctional impurities produced in step 2) is controlled such that the content of the polyfunctional impurities in the final product is 0.1 ppm to 1000 ppm. In the present invention, by respectively controlling the contents of monofunctional impurities and polyfunctional impurities in the cold reaction stage and the hot reaction stage, respectively, a precise control of the solvent consumption amount can be achieved, thereby greatly reducing the energy consumption.