The invention provides a method for reinforcing a concrete dam by caissons, relating to a method for reinforcing a concrete dam. In the reinforcing method, caissons are arranged in the front dam section of the dam needing to be reinforced, forming a caisson group; reinforced concrete caissons are used as the construction surface of riprap type rock-fill concrete, and the riprap type rock-fill concrete is used to quickly backfill box grids of the caissons and the connection between the caissons and the dam body, and conventional type rock-fill concrete is used to concrete up to the design elevation, and the front-dam reinforcement of the concrete dam is completed. The method does not need to empty a reservoir, thereby the construction is easy and convenient, and effects of improving the construction efficiency and saving the construction cost are obviously; moreover, the method has the advantages of high compaction degree of the formed riprap type concrete filling caisson structure, little cement amount, low hydration temperature rise, good volume stability, good impermeability, excellent comprehensive performance, energy conservation and environmental protection, etc.