The invention discloses modified phenolic resin capable of forming carbon nanotubes in situ and a preparation method thereof and belongs to the technical field of modified phenolic resin. The preparation method comprises uniformly stirring a phenolic compound, an aldehyde compound and a basic catalyst for a reaction to obtain methylol phenol, heating the methylol phenol, adding a transition metal hydroxide or oxide into methylol phenol, carrying out thermal insulation to obtain a phenolic resin intermediate product, adding an aminopolycanboxylic acid ligand into the phenolic resin intermediate product, carrying out thermal insulation, carrying out cooling to obtain aqueous modified phenolic resin, heating the aqueous modified phenolic resin in vacuum, and adding ethylene glycol or ethanol into the aqueous modified phenolic resin to obtain the modified phenolic resin capable of forming carbon nanotubes in situ. The preparation method can form high-yield carbon nanotubes at a temperature of greater than 800 DEG C in the inert atmosphere. The hydroxides or oxides of iron, cobalt, copper and nickel are used as the catalyst precursors. The modified phenolic resin has the characteristics of low cost, simple processes, no impurity and high catalytic efficiency.