In this paper, the influence on the pressure gradient of injection pattern has been researched by the laboratory physical simulation. Results show that, the pressure gradient of alternating injection pattern is higher than the general injection pattern in a wider range. In the period of alternating injection, the force which was generated by the pressure gradient is greater than the general injection pattern in a long period, and this force can make residual oil stressed and distorted. The impact on oil displacement effect of alternating slug frequency and slug combination has also been researched. Results show that, chemical recovery present the trend of first increasing and then decreasing with the increase of alternating slug frequency under the same usage of polymer, and the best alternate number is 4 times. The displacement effect of high concentration and high molecular weight slug combination is better than the low molecular weight and low concentration slug combination. When polymer injection amount is constant, the displacement effect of high molecular weight polymer and low molecular weight polymer alternately injecting is better than the high molecular weight polymer separately. The best displacement system: Main slug 0.1pv2000mg/L (25 million), deputy slug 0.25pv500mg/L（7 million）, alternate four times.