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For the thick sandstone reservoir, due to gravity differentiation, water drive mainly uses the bottom oil of the reservoir, while gas drive mainly drives the oil along the top of the reservoir. Water-alternating gas (WAG) injection can effectively combine the advantages of water flooding and gas injection, so that gas and water can work in synergy, thus further expanding swept volume and enhancing oil recovery. Over the past half century, the technology has been successfully applied to more than 60 oilfields worldwide, but its mechanism remains to be further studied. In this paper, a total of three tests were conducted through conventional water-flooding (WF), water flooding followed by gas flooding and WAG in sand-packed 2d-model. These experimental processes are recorded by video with time so that the saturation distribution of each phase in porous media can be observed，migration law of gas and water is studied, and the mechanism of WAG displacement is analyzed. The results show that gravity and capillary force have great influence on the process of gas water alternation, thus increasing the overall swept volume of water and gas; The water/gas alternating injection has the highest oil recovery factor (RF) of 75.45% in test 3, in comparison with water flooding followed by gas flooding (70.85% in test 2) and water-flooding (66.7%in test 1); Increase in cycles of WAG tends to reduce residual oil saturation.