The invention discloses a reasonable excavation method of a deep large foundation pit under water-rich round gravel conditions. The method comprises the steps that 1, excavation preparation is carriedout, particularly, a flood-proof wall is erected around the foundation pit, an annular drainage ditch communicating with a sedimentation tank is formed, construction layers are divided according to the construction depth, and construction sections are divided according to the construction area; 2, surface earth of the construction sections is excavated; 3, earthwork excavation is carried out on the first construction layer and the second construction layer; 4, earthwork excavation is carried out on the third construction layer and the fourth construction layer; 5, earthwork excavation is carried out on the fifth construction layer and the sixth construction layer; 6, earthwork excavation is carried out on the remaining construction layers, particularly, excavation is carried out circularly and gradually according to the steps from 3 to 5, remaining earthwork is excavated manually after excavation is carried out to 0.2-0.5m above the bottom of a foundation pit, and a concrete foundation cushion layer is poured; and 7, earthwork excavation is carried out on the rest construction sections, and particularly earthwork excavation is carried out section by section from the middle construction section to the construction sections on the two sides according to the steps from 2 to 6. The method can overcome the water-rich round gravel complex construction environment, reduce the foundation pit excavation risk and improve construction efficiency.