The invention discloses a preparation method of a high-temperature-resistant modified sulfonated phenol formaldehyde resin for drilling fluids, which comprises the following steps: 1) adding a phenol reactant, an aldehyde reactant and an acid catalyst into a reaction kettle, mixing, and reacting at 50-110 DEG C for 10-400 minutes; 2) adding an alkali catalyst and an aldehyde reactant into the reaction kettle, keeping the temperature at 50-110 DEG C, adding a sulfonating agent in batches, and reacting for 10-600 minutes; and 3) after the reaction finishes, cooling, discharging, and carrying out spray drying to obtain the product. The temperature resistance of the modified sulfonated phenol formaldehyde resin product can reach 200 DEG C. The high-temperature high-pressure filter loss of the drilling fluid under the aging condition of 200 DEG C can be effectively lowered as compared with the base mud, and the salt resistance can reach the salt concentration of 15% NaCl.