Based on the analysis of the observation we organized in August of 2000 off the Changjiag mouth,we found for the first time that there exists upwelling phenomenon at Lusi fish ground.The upwelling can bring the colder water and nutrient from the lower layer to the upper layer,and has an important effect on the environment and ecosystem.In order to understand the dynamic mechanism of the upwelling phenomenon,we used a three-dimension high resolution turbulent closure circulation model in spherical coordinates based on the modified ECOM_si model to study this upwelling.The calculated domain includes the East China Sea,Yellow Sea and Bohai Sea,and the real coastline and water depth were considered.In the case of considering all dynamic factors (water volume flux at the open boundaries,density gradient forcing and wind stress),the numerical model reconstructed successfully the upwelling phenomenon which is fairly consistent with the observations.The numerical experiments show that the dynamic mechanism of the upwelling is mainly produced by the baroclinic effect that is induced by the mixing between fresh water from the Changjiang River and the saline water.The wind stress has a little effect on it,and the water volume flux at open boundaries has no effect on it.The combination force of the slop of bottom with the baroclinic effect and wind stress makes the upwelling stronger.