The beet armyworm Spodoptera exigua(Hübner) is a serious insect pest and has caused serious economic losses in China in last decade. For further understanding the population dynamics and improving the forecast and control of this species, the DNA polymorphism of 7 geographic populations from various locations of mainland China was analyzed by using amplified fragment length polymorphism (AFLP) technique. The results showed that the maximum genetic similarity of S. exigua individuals between geographic populations was 0.824, and the minimum was 0.143;  the maximum genetic similarity of S. exigua individuals within geographic populations was 0.786 and the minimum was 0.250. The genetic similarity of some samples within geographic populations was lower than that of some samples between geographic populations. The genetic diversity of Jiangxi population was the lowest (the proportion of polymorphic bands was 80.7%) and that of Shandong population was the highest (the proportion of polymorphic bands was 88.6%). The genetic diversity of S. exigua from northern areas was higher than that from southern areas on the whole. The results of cluster analysis showed that 42 individuals of 7 populations did not relate to the geographic locations where they were collected, suggesting that the genetic differentiation is not obvious in all the populations studied. The northward and southward migration by the adult beet armyworm is the most possible explanation for these results, since other direct or indirect evidences has shown that it is a true migrant species in the country.