The invention discloses long carbon chain nylon modified phenolic resin and a preparation method thereof. The method comprises the following steps of: mixing long carbon chain nylon, phenol and an acid catalyst, heating to dissolve the long carbon chain nylon, dripping aqueous solution of formaldehyde with stirring, controlling the reaction temperature to be 94-102DEG C, reacting for 2 to 5 hoursafter the dripping is finished, purifying and drying, wherein the total number of carbon atoms in two carbon chains connected by an amido bond in the long carbon chain nylon is not less than 16. The method has the advantages of improving toughness of modified phenolic resin, reducing water absorbability of the modified phenolic resin, saving energy, reducing consumption, along with mild reaction conditions, narrow molecular weight distribution and moderate viscosity of the modified phenolic resin, no special requirement on production equipment and the like, and is suitable for industrial production.