The invention discloses flame retardant waterborne polyurethane which is prepared from the following raw materials: toluene diisocyanate, 2, 2-dimethylolpropionic acid, polyether diol, a catalyst, triethylamine, a silane coupling agent, deionized water, a defoamer and a dispersed nano-silicon sol. According to the flame retardant waterborne polyurethane disclosed by the invention, a polyurethane emulsion is modified by using the silane coupling agent, so that polyurethane has excellent waterproofness, polyurethane, mechanical property and the like, and moreover, the dispersed nano-silicon sol is introduced by using an ultrasonic dispersion method to prepare the flame retardant waterborne polyurethane. As SiO2 has reinforcing and flame-retarding effects, a SiO2 covering matter is formed when the polyurethane is combusted so as to have dual functions of shielding and insulating heat, alleviating burning rate and inhibiting combustion reaction, so that the flame retardant effect is improved, and moreover, the preparation method is good in operability and has a good application prospect.