The invention relates to a preparation method of a low acidic phenolic foam insulation material. Phenolic resins are prepared from raw materials of phenol, paraformaldehyde, a catalyst, a modifier, an acid capture agent, and then the low acidic phenolic foam insulation material is prepared from raw materials of the prepared phenolic resins, a foaming agent, a curing agent, a filling material, and a foam stabilizer. Compared with the prior art, the preparation method of the low acidic phenolic foam insulation material reduces the curing agent dosage, reduces the acidity of the phenolic foam insulation material, thereby improving the bonding intensity between the insulation materials and walls.