In this research, both experimental and numerical investigations are conducted to study the mechanism behind slug flow in a capillary tube. A 1.2 mm inner diameter capillary model is employed to represent the pore structure of porous media. A single oil slug is trapped in the tube and mobilized from a stationary condition by water injection. During the flow process, the flow behavior, the maximum driven pressure to mobilize the oil slug and the flow time to reach the maximum pressure are recorded and analyzed. The results show that the generation of a thin water film between the oil slug and the tube wall is essential in order to mobilize the oil slug from a stationary condition. During the process of water film generation, four different flow phenomena are observed: (1) Water film developing forward; (2) water film developing backward; (3) the leakage phenomenon; and (4) the oil slug breaking up. The appearance of these four phenomena is determined by both the oil slug length and the water injection velocity. Two models are created in the software package FLUENT. Based on the experimental settings, the flow behavior of the oil slug is simulated. The results indicate that both of these two models perform well in simulating oil slug shape variation. The impacts of diameter variation along the tube, water injection velocity and oil slug length are also simulated and analyzed. It can be summarized from the results that the maximum driven pressure magnitude is proportional to the water injection velocity and the oil slug length, and the flow time is inversely proportional to the water injection velocity. Quantitatively, the numerical results are consistent with the experimental results. This research imparts a better understanding of the mechanism behind oil slug flow. Moreover, it is a practical element for the further study of EOR technology.