The invention discloses an external wall neck construction method of the underground cast-in-situ concrete structure, which includes the following steps: (1) arranging reinforced bars, wall body external steel bars and wall body internal steel bars on a bedding course, arranging a first exterior sheathing composed of a multilayer board and a square timber outside the wall body external steel barsin the method of external suspended form supporting formwork; (2) arranging the first exterior sheathing composed of the multilayer board and the square timber inside and fixedly connecting with the wall body internal steel bars, arranging a water-stopping steel plate between the wall body external steel bars and the wall body internal steel bars; (3) casting the concrete and finishing the underground cast-in-situ concrete structure external wall neck construction. The method enables a calendering operation of the upper surface of a foundation plate through surface rolling mechanically or manually, and enables the wall foot of the wall neck to be conveniently rolled to achieve the process requirement of one-step operation of casting and rolling together and the effect of exposed concrete.The method is easy, practical and economic, and the flatness can be controlled.