The invention discloses a foundation pit construction method utilizing water pressure for support. The foundation pit construction method comprises the steps that S1, an enclosure structure is constructed around a foundation pit, and an underground water level at the site of the foundation pit is determined; S2, a water level in the foundation pit is adjusted according to the burial depth of the underground water level, so that the water level in the foundation pit is higher than the water level out of the foundation pit, and therefore, internal and external water level difference of the foundation pit is formed; S3, by utilizing the water level difference, water-earth pressure on the inner side of the enclosure structure of the foundation pit is higher than water-earth pressure on the outer side, and stress balance of the structure of the foundation pit is ensured; S4, foundation pit excavation is performed according to the internal and external water level difference which is determined through calculation; and during the foundation pit excavation process, the internal and external water pressure difference is maintained by gradually descending water or pouring water into the foundation pit until the pit is excavated to the bottom of the pit, and foundation of the foundation pit is finished. By adopting the method, internal and external water pressure difference is realized by reasonably utilizing water pressure to ensure stress balance of the foundation pit structure, and completely-support-free construction of the foundation pit is realized; and the problems of inner-pit space occupation, poor support effect and the like which are caused by utilizing anchor cables and other support schemes in the prior art are solved.