The invention discloses a GAT (GuAnTe) exterior formwork cast-in-place concrete composite insulation system and a construction technique thereof. The GAT exterior formwork cast-in-place concrete composite insulation system comprises a GAT composite insulation exterior formwork that is a dismantling-free formwork; concrete is cast between the GAT composite insulation exterior formwork and an interior formwork to form a cast-in-place concrete wall; the GAT composite insulation exterior formwork and the cast-in-place concrete wall are fixedly connected via connectors. The construction technique includes layout, setting out, cutting, mounting of special connectors, tying of rebars and pads, erection of the GAT composite insulation exterior formwork, erection of the interior formwork, fitting of tie bolts, erection of formwork wooden auxiliary battens and dual-steel-tube main battens, position adjustment for formwork fixation, casting of concrete, removal of the interior formwork and the main and auxiliary battens, splicing and internal and external corner anti-cracking treatment, leveling and mortar construction, and facing layer construction. The GAT exterior formwork cast-in-place concrete composite insulation system and the construction technique thereof have the advantages that the system herein has better thermal insulation, fire resistance, weather resistance, impact resistance and freeze thawing resistance and is good in safety, well applicable, economical and reasonable, engineering quality is guaranteed at the premise of construction period shortened, energy is saved,and emission is reduced.