The invention relates to an enclosure structure of a pit in a pit and a construction method thereof. The method comprises the following steps that enclosure piles are driven and arranged along the edge of the to-be-excavated pit in the pit; earth excavation is performed so as to form the pit in the pit, and the sides, close to the pit in the pit, of the enclosure piles are exposed; and prefabricated slabs are provided, the prefabricated slabs are vertically arranged along the edge of the pit in the pit and are attached and fixed to the sides, close to the pit in the pit, of the enclosure piles, the bottoms of the prefabricated slabs are inserted into the pit bottom of the pit in the pit, and the tops of the prefabricated slabs are fixedly connected with the enclosure piles. According to the enclosure structure of the pit in the pit and the construction method thereof, through the prefabricated slabs, the situation that gaps between the enclosure piles are relatively large, so that running sand is prone to rush into the pit in the pit is prevented, the construction of waterproof coiled materials is facilitated, moreover, the construction progress of the pit in the pit is accelerated, the dangerous situation of the pit in the pit is effectively prevented, and the pit in the pit can be quickly sealed.