The Qiongdongnan basin LING A in the South China Sea is a complex fault block oil and gas field,which is characterized by complicated fault block in deep strata and great changes in vertical and horizontal velocity as well as low S/R.All of these factors lead to an unreliable imaging results.This situation seriously restrict the residual oil exploration.The key to solve this problem is to improve the accuracy of PSDM imaging with high precision velocity modeling.This paper presents a high precision 3D velocity modeling method based on geological constraint.On the basis of tomographic velocity modeling,the horizon and fault interpretation are used to constrain tomography inversion.This method designs a new strategy to update the velocity model,and improves the construction of initial model.The 3D velocity field is improved by new workflow,and the quality of profile of PSDM imaging is also enhanced.The results obtained by the authors confirm that the new velocity modeling method can provide the basic data for the interpretation of fine horizon and implementation of fault block.