An underwater riprap foundation which can be constructed efficiently using a reduced amount of ripraps and a consolidation smoothing method for efficient construction of the same are disclosed. The underwater riprap foundation includes a first region for supporting comparatively high load from a structure such as a caisson, and a second region positioned adjacent the first region for supporting comparatively low load from mulch blocks or the like. The first region is formed as a rigid riprap core wall body having a consolidation density higher than that in the second region. The first and second regions have top faces flush with each other. In construction, ripraps are thrown into water such that the extra-banking height in the first region is higher than in the second region, and free fall of a weight is repetitively carried out to the first and second regions to consolidate the ripraps.