The invention relates to a preparation method of coupling phenolic resin used for a drilling fluid, and the method comprises the following steps: successively adding 100 parts of phenol and 60 parts to 200 parts of formaldehyde according to mass ratio into a reaction vessel with a backflow cooler of a mixer, heating to 80 - 100 DEG C, stirring and reacting for 30 - 50 minutes under the effect of 1 - 3 parts of A type catalyst, then adding 10 parts - 20 parts of B type catalyst and 150 parts - 200 parts of sulfonate to heat at the temperature range of 90 DEG C - 105 DEG C to react for 1 - 2 hours, then adding 50 - 100 parts of aminosilane coupling type high-molecular polymer, and reheating to the temperature range of 90 DEG C - 105 DEG C for continuing the reaction for 2 - 4 hours, and then discharging the material to obtain coupling phenolic resin. According to the invention, when coupling phenolic resin is taken as a fluid loss agent with heat and salinity tolerance used for the drilling fluid, the high-temperature high-pressure filtration loss is reduced by 8 - 15%, the cloud point salinity is raised by 10 - 20% than that of common sulfonation phenolic resin on the key index tests, and the quality of the filter cake is raised so that the filter cake is thin and dense.