The invention provides a method to prepare polymethylene polyphenyl polyamine(called polyamine, DAM for short). The method takes a revolving packed bed as a reactor for the mixture of formaldehyde and aniline hydrochloride to react. Aniline hydrochloride, recycling solution mixture and formaldehyde are fed to the revolving packed bed proportionally and are mixed under hypergravity and undergo condensation reaction; the material running off the revolving packed bed enters a stirred tank for pre-condensation reaction to obtain condensed solution; then the condensed solution is heated and processed with molecular rearrangement, neutralization, bath and refining, finally getting refined DAM. With the method of the invention, key secondary reaction resultant during the condensation process is remarkably reduced; the circulation loop and the inner wall of the heat exchanger will not be adhibited with or blocked by sediment; the refined DAM is low in impurity content and the subsequent product MDI is of light color and stable improved quality.