The invention relates to the field of building construction, in particular to a construction method for reinforcing an underground outer wall formwork in a complex limited space in a foundation pit. The construction method for reinforcing the underground outer wall formwork in the complex limited space in the foundation pit comprises the following steps of manufacturing a concrete prefabricated outer wall formwork, wherein the height is the floor height, grouting sleeves are arranged at the bottom, a plurality of bolts are pre-buried, and matched screw holes are formed in an inner side formwork; pre-burying a row of vertical steel bars on the outer side of an underground outer wall, wherein the distance of the vertical steel bars is consistent with the distance of the grouting sleeves; hoisting the prefabricated outer wall formwork, enabling the grouting sleeves to be aligned with raft vertical steel bars, adjusting the prefabricated outer wall formwork to be perpendicular to the horizontal plane, and fixing through an inclined support; conducting grouting through the grouting sleeves; binding concrete wall steel bars; dismantling the inclined support, erecting an inner side formwork, enabling the bolts to penetrate through the screw holes, and screwing the nuts for reinforcement; pouring a concrete wall body; and after final setting, dismantling the inner side formwork, then dismantling the exposed parts of the bolts, and repairing the surface of the concrete wall body. The construction method for reinforcing the underground outer wall formwork in the complex limited space in the foundation pit provided by the invention solves the problem that when an operation surface is insufficient, constructors cannot reinforce the underground outer wall structure formwork.