Conglomerate reservoir has typical complex mode pore structure，strong heterogeneity and is easy to form dominant channels of different grades；hence，it is difficult to achieve the effect of expanding sweep volume by single slug and general injection method. Three-pipe parallel core displacement experiments with different injection modes，including single constant viscosity，step-by-step viscosity reduction and step-by-step viscosity increase，was carried out and the enhanced oil recovery（EOR）were investigated under the same polymer dosage. In the best injection mode，three-pipe parallel core displacement experiments were carried out at different injection cycles and injection rates. The experimental results showed that，the EOR could be increased by 13.61% by using step-by-step viscosity reduction injection mode，which was 2.31% higher than that by single constant viscosity injection mode. Under the same conditions，the EOR was not related to the number of injection cycles. At the reasonable injection rate of 1.5 mL/min，the EOR could be further increased. When EOR was similar，the polymer dosage by step-by-step viscosity reduction mode was 50% lower than that by single constant viscosity injection mode. The results of the step-by-step viscosity reduction in the filed indicated the oil production was increased and the water cut was decreased obviously.