A function polymer is considered to be a kind of potential Enhanced Oil Recovery (EOR) agent that can further enhance oil recovery after polymer flooding. Based on the typical geologic features and development history of the Daqing Oilfield, a function polymer displacement experiment was conducted using a 3D laboratory physical model with a high-permeability channel between the injection end and the production end to study the EOR mechanism of the function polymer solution. In this article, the displacement feature and oil/water distribution of function polymer flooding is presented, as well as the swept effect in pores of different sizes. The experimental results show that the function polymer flooding after regular polymer flooding further enhances oil recovery by 10.29%. Moreover, the increment of sweep efficiency and displacement efficiency reaches 3.02% and 11.78%, respectively, which clearly demonstrates that the function polymer system improves the displacement efficiency by the emulsification effect and increases the displacement efficiency by the plugging effect. Combined with nuclear magnetic resonance imaging, the swept effect in pores of medium size tends to occupy the dominate role in the performance of function polymer flooding, rather than that in pores of smaller or larger size. By analyzing the EOR mechanism of the function polymer system, this work offers a novel theoretical reference for future decision-making in tertiary recovery.