Abstract: Alfalfa is an important forage in Northwest China but in recent years, several pests have seriously damaged alfalfa. To monitor the dynamics of pest populations, two typical regions in which alfalfa is the dominant crop were selected. The five-spot method was used to sample pests and natural enemies in alfalfa fields. The community structure of insects was stable with an increase in age of alfalfa crops. The relationships between pests and natural enemies in two monitoring sites were fitted with four equations: V₁=0.532X₁+0.326X₂+0.035X₃, W₁=0.108Y₁+0.482Y₂+0.035Y₃-0.113Y₄, V₁=0.356X₁-0.163X₂+0.008X₃, W₁=0.063Y₁+0.526Y₂+0.103Y₃-0.038Y₄. In addition, natural enemies played an important role in controlling pest population in these alfalfa fields. Sucking insects (aphids) had a high abundance but were attacked by a large number of natural enemies in alfalfa fields. Leaf eating pests, root pests, and flower eating pests had the potential to cause great damage to alfalfa grasslands, because of lack of natural enemies. Natural enemies had wider niches than pests, indicating that natural enemies needed to search for prey on a large spatial scale. Ladybirds were the dominant natural enemy, and had a similar niche to pest aphids. This work supplied an important basis and theory for integrating pest management and sustainable development of the alfalfa industry.