The invention relates to a vegetable tannin extract modified phenol formaldehyde foam and a preparation method thereof. The vegetable tannin extract modified phenol formaldehyde foam is prepared from the following raw materials in parts by weight: 100 parts of vegetable tannin extract modified phenol formaldehyde resin, 5-8 parts of surfactant, 3-9 parts of foaming agent and 15-20 parts of curing agent, wherein the vegetable tannin extract modified phenol formaldehyde resin is prepared from the following raw materials in parts by weight: 100 parts of phenol, 30-50 parts of vegetable tannin extract degradation liquid, 61-73 parts of polyformaldehyde, 5-7 parts of urea, 1-3 parts of melamine and 8-12 parts of ethylene glycol; and the vegetable tannin extract degradation liquid is degraded vegetable tannin extract obtained by performing degradation pretreatment on vegetable tannin extract with oxydol under alkaline conditions. According to the invention, on the premise of not influencing the own flame retardancy and heat-insulating property of the phenol formaldehyde foam, the pull strength and bending failure force of the phenol formaldehyde foam are obviously improved, and the free formaldehyde content in the phenol formaldehyde foam is obviously reduced. The phenol formaldehyde foam can be used as a heat isolation, heat insulation and fireproofing material for buildings, and is a product having favorable market prospects.