The invention discloses a kind of LBL self assembly polyelectrolyte nano flame retardant coating and preparation method thereof, comprise the following steps: 1) pretreatment: matrix material is put into acid solution and soaks, with dry after water cleaning; 2) self assembly: pretreated matrix material is put into cationic polyelectrolyte solution and soaks, after forming one deck cationic membrane, dry after taking out the cleaning of use water, then put into anionic polyelectrolyte solution and soak, with dry after water cleaning after formation one deck anionic membrane; Repeat above-mentioned self assembling process successively repeatedly, namely obtain LBL self assembly polyelectrolyte nano flame retardant coating at substrate material surface.The present invention adopts LBL self assembly technology to prepare polyelectrolyte nano flame retardant coating, environmental protection, and preparation method is simple, does not destroy the internal structure of matrix material, there is higher flame retarding efficiency simultaneously, be applicable to form flame retardant coating on fabric, timber, foam, plastic or other material surface.