The invention discloses a dewatering reinforcement construction method for a water-rich fine sand stratum in an enclosed space. The construction method comprises the following steps that 1) drilling construction is conducted on the diaphragm wall at the newly-arranged storey-adding transfer joint below an existing subway station, the drilling construction comprises detection hole construction, grouting hole construction and dewatering hole construction, grouting holes are drilled in the upper area of the corresponding diaphragm wall in a layered mode, and dewatering holes are drilled in the lower area of the corresponding diaphragm wall in a layered mode and serve as later grouting holes; (2) a pore water pressure gauge and a downcast pipe are arranged; and (3) dewatering reinforcing operation: the dewatering reinforcing operation is divided into an early dewatering reinforcing stage, a middle dewatering reinforcing stage and a later dewatering reinforcing stage. According to the invention, the layered synchronous implementation of grouting and dewatering is realized, and meanwhile, the pore water pressure gauge is adopted to monitor the dewatering and grouting pressure and keep the pressure balanced, so that the effect of quickly finishing dewatering for reinforcing the soil body in the closed space at the storey-adding transfer node under the condition that the operation of the existing subway station is not influenced is achieved.