The invention discloses heterogeneous nucleating agent toughening-modified phenolic resin foam and a preparation method thereof. The preparation method of the foam comprises the following steps of preparing a heterogeneous nucleating agent and preparing the modified phenolic resin foam. According to the method, silane coupling agent modified graphene is added for regulating the foam pore structure; meanwhile, the synergistic effect of graphene nanoparticles is exerted, the defects that phenolic foam is poor in mechanical property, high in brittleness and prone to disengagement are effectively overcome, and wide application of the phenolic foam in the engineering field is promoted.