In order to improve the phenomenon that the producing degree of Pu Ⅰ 1—3 layers was uneven and the polymer circulated along high permeability layers inefficiently in Xingshugang oilfield，three tubes parallel core injection experiment was designed. The impact of olymer relative molecular mass，concentration and injection mode（single injection and alternative injection）on the recovery was researched，and the alternative injection cycle was optimized using numerical simulation method. The results showed that with increasing relative molecular mass and concentration of polymer，the core recovery increased. Single polymer could effectively flood the high permeability layer，but had poor effect in low permeability layer. The average recovery ratio of alternative injection polymer flooding was 3.3%—5.47% higher than that of single injection polymer flooding. The oil displacement effect of polymer flooding with first injection of high molecular weight polymer slug and then small molecular weight of polymer slug was better than that of reverse injection order polymer slug. The numerical simulation results showed that it was not conducive to control fluidity if the alternate injection interval was too long，while it was easy to cause viscosity loss if the time interval was too short，which resulted in the loss of a large amount of energy and was not conducive to oil displacement. So the best alternative injection period was 3 months，which could increase the recovery ratio by 11.68%.