The invention relates to halogen free phosphorous-nitrogen synergic antiflaming water-soluble polyurethane fabric coating agent and a preparation method thereof, belonging to the technical field of light industry. The coating agent comprises diisocyanate, non-antiflaming polyalcohol, reactive type phosphorous-nitrogen series polyalcohol, hydrophilic chain extender and neutralizing agent. The diisocyanate, non-antiflaming polyalcohol, reactive type phosphorous-nitrogen series polyalcohol and hydrophilic chain extender are added into a kettle, and reaction is carried out, thus obtaining prepolymer; then the prepolymer is added into aqueous solution containing neutralizing agent, emulsification neutralization is carried out in high speed emulsification equipment, and then rotary evaporation is carried out to remove solvent, thus obtaining target product. The invention has the characteristics of environmental protection, no toxicity, low smoke, phosphorous-nitrogen synergic antiflaming and obvious antiflaming effect; the antiflaming water-soluble polyurethane fabric coating agent is adopted for finishing fabric, coating is transparent and unsticky, handfeel of fabric is not influenced, and soaping resistance can be realized, thus passing level B1 in GB/T5455-1997 textile-burning behaviour-vertical method.