The invention relates to the technical field of reinforced resin, in particular to a lightweight sound insulation phenolic resin. The lightweight sound insulation phenolic resin comprises a componentA, a component B, a component C and a component D that are packaged in mutual independence. In terms of 100 weight parts of a phenol raw material, the component A comprises: 10-30 weight parts of a first polymer emulsion and 0.6-10 weight parts of a foaming agent; the component B includes: 18-56 weight parts of a second polymer emulsion, 1-6 weight parts of a soluble metal salt, and 10-30 weight parts of a chain extender; the component C includes: 100 weight parts of a phenol raw material, 40-120 weight parts of an aldehyde raw material and 10-30 weight parts of a catalyst; and the component Dcomprises: 200-400 weight parts of an organic solvent. According to the invention, adding of composite fiber with a multiple pore structure into phenolic resin can reduce the mass of the system, block the propagation of sound and reach a soundproof effect. In addition, the composite fiber is composed of a dual structure of inner core and outer wall, has high strength and toughness, and can reinforce phenolic resin.