The beet armyworm, Spodoptera exigua (Lepidiptera: Noctuidae), is an important agricultural pest in China, causing serious economic losses in the main welsh onion-producing areas of North China. In order to effectively monitor and manage this pest, it is necessary to investigate its genetic variation and intraspecific phylogeographic structure in North China. In this study, we used a DNA fragment of cytochrome oxidase subunit I (COI) consisting of 518 bps to investigate its genetic diversity. A total of 500 individuals of this species were collected from 20 geographic locations, and 8 haplotypes were identified. Low levels of genetic diversity (h = 0.256 ± 0.025, π = 0.00058 ± 0.00008) for the total populations in these study regions were obtained. Phylogenetic and median-joining network analyses indicated that there was no distinct geographic distribution pattern among haplotypes. The results also indicated that the mean pairwise sequence difference between the populations was 0.1%, ranged from zero to 0.2%. The genetic differentiation (Fst) between the populations varied from −0.198 to 0.656, and there was a significant differentiation between the population of Kunshan (KUNS), Shenyang (SY), Lucheng (LUC), Binzhou (BZ) and the remaining populations, respectively. The analysis of molecular variance (amova) showed that the percentage of variation within a population (69.05%) was greater than that between the populations (30.95%), and the level of population differentiation (FST = 0.310, P < 0.001) was highly significant after 1000 random permutations. Finally, unimodal mismatch distribution combined with negative, Tajima's D (D = −1.956, P < 0.05) and Fu's Fs (Fs = −5.875, P > 0.05) indicated that S. exigua experienced recent population expansion in North China.