The invention discloses phosphorus-nitrogen-containing DOPO (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide) derivative flame retardant as well as a preparation method and application thereof. Thephosphorus-nitrogen containing DOPO derivative flame retardant has a structural formula shown in the description, in the formula, R1, R2 and R3 are respectively independently selected from hydrogen,methyl, ethyl, propyl and butyl; and R4 is selected from ethyl, propyl and butyl. The flame retardant synthesized by the invention has a molecular structure with an annular structure of large rigidity, is good in thermal stability, meanwhile comprises phosphorus and nitrogen of which flame retardance functions can be brought into play, and thus has a good flame retardant effect.