Objective To understand the underweight status of children under 3 years old in 14 counties in the western areas of China and to explore the relative factors so as to provide reasonable suggestions for improving the nutrition status of children. Methods With probability-proportional-to-size sampling,2 999 caregivers were interviewed and the weight of 3 051 children under 3 years old were measured during September and October of 2011.Whether the child was underweight or not was judged by the WHO evaluation standard published in 2006. Results The prevalence of underweight among children under 3 years old in the survey area was 5.8%.The results suggested that low household income (χ²=30.729,P<0.001),living in Ⅳ rural region (OR=4.14,95%CI:1.88~9.14),insufficient dietary intake diversity (OR=1.63,95%CI:1.09~2.43) were the risk factors of child underweight. Conclusions The prevalence of underweight of children in the surveyed areas is improved comparing with the results from previous studies,but the prevalence is still at a high level.Improving the local economic level and popularizing knowledge of scientific feeding is the key to reduce the prevalence rate of underweight of children.