The invention discloses a phenolic foam thermal-insulation material of a flat-plate solar collector. The phenolic foam thermal-insulation material is prepared by phenolic resin, flame retardant, foaming agent, curing agent, stabilizer, toughening agent and chitosan according to weight through mixing, foaming and curing. The phenolic foam thermal-insulation material has the advantages that the phenolic foam thermal-insulation material is applied to solar water heaters, the problem such as water tank shrinkage due to the fact that polyurethane foam is used as the thermal-insulation material can be solved effectively, the thermal insulation effect can be further increased by the added chitosan, and the phenolic foam thermal-insulation material is fireproof, flame-retardant, wide in working temperature, energy-saving and environmentally friendly.