A petroleum drilling additive high temperature-resistant filtrate reducer and a preparation method thereof. The filtrate reducer is prepared by the following raw materials by weight: a humic acid-containing substance, sodium pyrosulfite, water, organic silicon resin, a cross-linking agent, potassium hydroxide, AMPS, alpha-methylglucoside. The preparation method of the filtrate reducer comprises the following steps: weighing water, potassium hydroxide, lignite or peat or weathered coal, sodium pyrosulfite according to the weight parts, adding into a stirring reaction vessel, sealing, heating, reacting, cooling; adding organic silicon resins, cross-linking agents, AMPS, alpha-methylglucoside weighed according to the weight parts into the stirring reaction vessel, sealing, heating, stirring and reacting; discharging the materials, drying the product to obtain a finished product. The finished product can resist a high temperature of 220 DEG C, can resist saturated brine and seawater, effectively reduces drilling mud filtration loss, does not cause tackification, and thus solves the problems of temperature resistance and salt resistance of mud in deep well, ultra-deep well drilling.