The invention discloses an assembling type template for reinforcing a square cross-section column and a construction method of the assembling type template. The assembling type template for reinforcing the square cross-section column is characterized by comprising standard template sections, a column top template section, fastening bolts and connecting inserting plates. Upper and lower layers of adjacent templates are in butt joint in a staggered manner, and the template which is used as a part of a reinforcing structure is not dismounted after construction is finished. The template can be prefabricated in a factory, can be used as a column body bearing structure, has the characteristics of light self weight, high strength and good integrality, and is suitable for rebuilding, reinforcing and maintaining old buildings, a construction process can be simplified, working hours are shortened, and the adaptability is high.