The wettability of a porous medium had significant effects on microscopic and macroscopic displacement efficiency. Visual microscopic model and image processing technique were conducted to investigate displacement efficiency and displacement mechanism of polymer flooding. The effect of micro aspect and distribution of bound water and heavy oil on polymer solution adsorption, trapping, continuous and discontinuous flowing, displacement frontier and pore to pore sweep oil had been analyzed under strongly oil-wet and water-wet medium. The results conducted by microscopic model indicated that in water-wet medium, the formation water tended to distribute in the surface of rock skeleton and form thick water film near the wall of hole. Also, polymer tended to adhere to the wall of pore hole which had higher sweep efficiency. On the contrary, in oil-wet medium, bound water mainly distributed to the porous medium in continuous phase and crude oil absorbed on the pore throat. Snap-off effect of polymer solution resulted in a small part of sweep oil, and emulsion formed by oil and polymer solution assisted to polymer flooding. Microscopic displacement efficiency of polymer flooding in water-wet medium was apparently higher than that in oil-wet medium. The experiment results of core flooding accorded with that of microscopic model. On the one hand, longer frontier breakthrough time was observed in strong water-wet cores and the breakthrough leading edge was more uniform with 21.5%OOIP oil recovery for water flooding, compared with oil-wet ones (15%OOIP). On the other hand, in oil-wet medium, the threshold of polymer viscosity was higher than that in water-wet medium. After water flooding, 23%OOIP and 17%OOIP oil recovery of water-wet and oil-wet cores was improved by 500 mPa·s polymer flooding respectively.