A numerical simulation is performed using the Princeton Ocean Model (POM) to investigate the behavior of the Changjiang Diluted Water (CDW), and to evaluate the freshwater budget in the Yellow and East China Seas. In addition, the major process controlling the behavior of the CDW is discussed. The simulated CDW distributions are fairly consistent with the observed distributions, showing a remarkable seasonal variation. The CDW extends to the northeast toward the Tsushima/Korea Straits during summer, while it hugs the Chinese coast to the southwest within a narrow band in winter. Tracer experiment is carried out to track the freshwater originating only from the Changjiang, and to evaluate the freshwater budget in the Yellow and East China Seas. The result shows that about 68 percent of the Changjiang discharge is transported through the Tsushima/Korea Straits, and that about 27 percent is transported through the Taiwan Strait in autumn and winter. To investigate the main process controlling the behavior of the CDW mentioned above, another experiment is carried out without wind-forcing. It is found that the eastward extension of the CDW is strongly constrained by the Taiwan-Tsushima Warm Current System over the shelf regime. The wind is, however, responsible for the northeastward extension of the CDW in summer. In addition, the southwestward extension at the Taiwan Strait during winter is done by strong northeasterly monsoon. The experiment without wind-forcing shows that freshwater transport through the Tsushima/Korea Straits increases to 90 percent of the total Changjiang discharge.