The invention discloses an in-situ polymerized flame retardant. The in-situ polymerized flame retardant is derived from a diacid monomer A and a diamine monomer B. A1 is terephthalic acid, A2 is a reactive flame retardant diacid monomer containing an aromatic ring in a phosphorus system, A1+A2=100 mol%, A1=50-90 mol%, and A2=10-50 mol%. The diamine monomer B is one or more of diamine monomers containing 4-36 carbon atoms. According to the invention, the novel flame retardant is obtained by in-situ polymerization of phosphorus series reactive flame retardant diacid monomers containing aromaticrings in semi-aromatic polyamide oligomers. The flame retardant of the invention has the advantages of no precipitation in the semi-aromatic polyamides, no influence on other properties of the semi-aromatic polyamides, and excellent flame retardancy.