Abstract: This paper compares the effect of continuous injection and alternating injection of polymers with different viscoelasticities on stress and deformation of residual oil droplets, explores the hydrodynamic displacement mechanism of them on residual oil, including the stress and deformation calculation of residual oil droplets under the actions of the different fluids, and analyzes the alternating injection mechanism from two aspects of the elasticity and pressure gradient. The results show that the difference between normal deviator and horizontal stress of residual oil droplets by the effect of polymer solution is increasing with the elasticity rise of the polymers in the case of the same pressure gradient. Comparing with continuous injection process, the alternating injection process of polymer solutions can improve the phenomenon of fluid overload in the large pores and the fluid deficiency in the small pores, making the polymer of low concentration and low molecular weight enter into the low permeability layers with more residual oil displacement and stress, hence the swept volume can be expanded and the oil displacement efficiency can be improved