The invention discloses a nitrogen and phosphorus-containing cross-linked polymeric flame retardant with a char forming function. A cross-linking point structure is as shown in the formula (I); and the invention discloses the nitrogen and phosphorus-containing cross-linked polymeric flame retardant with the char forming function. The cross-linked polymeric flame retardant has the characteristics of high flame resistance, high molecular weight, migration resistance, moisture resistance and high temperature resistance, does not affect mechanical properties of a substrate material, can be used independently and can also cooperate with a phosphorus-nitrogen-series flame retardant, and is applied to the materials, such as nylon, polyester and polyurethane.