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There are three kinds of typical residual oil after water flooding, which are trapped in dead end, trapped at pore throat and adhered to reservoir pores wall. According to the main feature of pore structure and the fundamental morphological feature of residual oil, the corresponding displacement models are put forward. Based on the numerical results of viscoelastic polymer solution flow in the displacement models, microscale displacement mechanisms are explored. Results show that for residual oil trapped in dead end, viscoelastic polymer solution can enhance displacement efficiency, as a result of flow field evolving in dead end more deeply with greater viscoelastic vortex. For residual oil trapped at pore throat, one displacement mechanism is that vortex developing to the deep of throat and decrease the Size of oil ganglia; the other is that the drive force, that is pressure drop on oil ganglia, overcomes capillary force, consequently it caused flow direction changes and fluid enter smaller throat. For residual oil adhered to reservoir pores wall, there is start-up interfacial tension, and oil displacement efficiency depends on viscoelasticity and oil property.