The invention belongs to the technical field of continuous steel structure aqueduct construction and particularly relates to a construction method of a heat preservation board for a box girder of a continuous steel structure aqueduct. Through the construction method of the heat preservation board for the box girder of the continuous steel structure aqueduct, the problem that a body of a box girder of a continuous steel structure aqueduct is prone to deformation and consequently safety accidents are caused is solved. The heat preservation board comprises a plastering mortar layer, an XPS heat preservation board layer and a bonding mortar layer from top to bottom. The plastering mortar layer internally contains at least two glass fiber nets. The XPS heat preservation board layer is formed by splicing a plurality of XPS heat preservation board bodies through staggered joints. The heat preservation board of a three-layer structure is tightly attached to the body of the box girder, so that the heat preservation board plays an effective heat preservation role on the box girder. The heat preservation board can also be applicable to special structures such as box girder pull-push openings, vent holes and drainage channels.