The invention discloses a high-temperature resistant polymer fluid loss agent for drilling fluid and a preparation method thereof. The polymer fluid loss agent is characterized in that the polymer fluid loss agent is a copolymer generated by olefine acid, alkenyl amide, alkenyl azululanone and alkenyl quaternary ammonium through reaction, molecules of the copolymer contain 800 to 1,800 structuralunits, and viscosity average molecular weight is 500,000 to 1,200,000. The polymer fluid loss agent can resist temperature of more than 245 DEG C in the drilling liquid, has good protective colloid performance to the drilling liquid, improves the quality of a filter cake, has good fluid loss reduction effect in fresh water, aggravated fresh water, salt water and the like, can simultaneously inhibit hydration and dispersion of clay and has good rheology.