Abstract: The WS-A Oil Field is a newly discovered one in the western basin of the South China Sea, which is mainly accumulated in a near-provenance fan deltaic depositional system. The productivity and pressure testing data of the evaluation well demonstrates that the reservoir of the field is complex in flowing characteristics, and the production capacity is poorly related with the reservoir physical property from logging interpretation, and thus the conventional method is not able to accurately characterize the flowing characteristics of the reservoir. Therefore, the concept of flow unit is introduced in this article. Since such a case is relatively scarce in offshore oil fields, we have to classify the flow units upon the basis of both the micro and macro characteristics of pore throat structure. Sorting of the sediments, electrical properties and porosity parameters are selected from among the others as the criteria for flow unit classification and production capacity evaluation. Evaluation results are well correlated with the measured data, suggesting that this method can accurately and effectively evaluate the complex reservoir as such and guide the oilfield development at present and in the future.