The invention relates to a feedback control method for crystal product granularity in the dilution crystallization process. The method includes the specific steps: estimating the sum of crystal granules according to expected average grain diameter of crystal products, comparing the sum of crystal granules N in crystallization mother liquor measured through an on-line granularity measurement apparatus in real time with the sum of crystal granules Ns specified in production, and synchronously controlling feeding rate of a crystal breaking system, dilution agent and solvent till the crystal products with expected granularity distribution are produced. Compared with the conventional granularity control method in the dilution crystallization process, the method has the advantages that the crystal products with expected granularity distribution can be produced automatically and conveniently as required by only regulating one parameter of the specified sum of crystal granules Ns without re-optimizing new process parameters and the grinding and screening process through plenty of experimental operation, and a granularity controllable regulation function of producing the crystal products with specified granularity distribution as required is realized.