Based on the investigating undergrowth vegetation in the forests with different densities in semi-arid region on the loess plateaus,the niche of undergrowth vegetation are described and analyzed,with different densities as the resource state and with the important value as the resource state descriptor of the niche.The results indicate that most of undergrowth vegetation have lower niche breadth values,but Heteropasppus altaicus and Arremisia sacrorum have higher niche breadth values,59.7% of the herbage species have niche overlaps,but which are lower.All the above reveal that the bad condition of undergrowth vegetations.