The invention relates to a joint seam structure of a building outer wall heat insulation board. An outer wall of a building is sequentially provided with a leveling layer, an adhesive layer, a composite heat insulation board and a veneer layer. The composite heat insulation board is a phenol foam plastic board with a grade-A combustion performance, a bayonet is respectively arranged on the edges of the composite heat insulation board, the surface of each bayonet is a composite heat insulation waterproof layer, foam polyurethane is filled in a gap between every two bayonets in the field, and building sealing paste is used for sealing the outside of the seam. By adopting the structure, since bayonets on the edges of the composite heat insulation board can prevent the permeable water to permeate the wall body, the heat insulation effect of the building is improved.