The study is carried out at Liulin and Ziaoyuan towns of Yan'an city in the loess plateau,Shaanxi province in July to September 2003.The vegetation coverage and zoker density were surveyed in seventeen one plots by random sampling.The result showed that there is a extreme significantly negative correlation between them.The higher vegetition coverage,the lower zoker's density as well as the rodent damage.The ecological threshold limit value of the vegetation coverage for forest growing safely is 64.14%.The time alignment during the vegetation renew and the regularity of growth and decline of the zoker's population on forestry-land there from the farmland in loess plateau is showed.