The invention discloses aminotrimethylene phoshonic acid and a preparation method and application thereof. The structure of the aminotrimethylene phoshonic acid is shown in formula (I), the prepared aminotrimethylene phoshonic acid is high in phosphorus content, has good heat stability, high flame retardant efficiency and a high char formation rate, does not contain halogen, is green and environment friendly, and can be widely used in the fields of polymer materials, wood, paper and like as a phosphorus-nitrogen flame retardant.