The beet armyworm, Spodoptera exigua (Lepidoptera: Noctuidae), is an important and cosmopolitan pest that attacks cultivated crops, including maize, cotton, soybeans, beet, tomato, cabbage, and alfalfa, causing serious economic losses in some of the main crop-producing areas. This species originated in South Asia and is distributed throughout the tropical and temperate regions of Asia, Europe, Africa, and North America. In China, S. exigua was first recorded in Beijing in the 1890s and has been widely distributed in the southern provinces in recent years. However, recently, with global climate warming and the adjustment of agricultural planting structures, S. exigua has quickly spread to the main crop-producing areas of North China. Genetic diversity and population genetic structure are important aspects of the population genetics of agricultural insects, and provide essential information for understanding local adaptation and dispersal patterns, and for clarifying the relationships between genetic variation and outbreaks of pest species. Cytochrome b (Cytb) has a moderate evolutionary rate and a clear evolutionary pattern, suitable for the studies of phylogenetic evolution at the intra- and inter-specific levels. To identify the genetic diversity and population history among geographic populations of S. exigua in North China, and clarify its population demographic history and genetic structure, the genetic diversity, structure, differentiation, and molecular variance were analyzed using DnaSP 5.0 and Arlequin 3.0. In the present study, 798 bases of mtDNA Cytb genes were obtained from 304 individuals of S. exigua, collected from 17 geographic populations in 8 provinces of North China in 2012. Of the 798 characters, 776 sites were conserved and 22 sites were variable (2.8% of the total length), including 9 parsimonious informative sites. The average base frequencies were 42.7% A, 33.1% T, 12.6% G, and 11.5% C. Within-locality diversity was estimated in terms of haplotype diversity (h) and nucleotide diversity (π) from all surveyed localities. Low genetic diversity (Hd = 0.422 ± 0.035,π = 0.00119 ± 0.00011) in the total populations among different geographic populations was detected. The highest estimate (Hd = 0.643 ± 0.106, π = 0.00214 ± 0.00039) was detected in Binzhou (BZ). Nineteen haplotypes, including 9-shared haplotypes were identified. Hap6 was shared in 228 individuals of all populations. Phylogenetic analysis was conducted to determine the relationships between S. exigua haplotypes, and detect discernible groups related to geographic distribution. High congruence was observed between the phylogenies derived from Maximum parsimony (MP), Maximum-likelihood (ML) and Bayesian analyses, and these analyses generated only two inclusive clades. Moreover, the median-joining network was similar to the topology of the phylogenetic tree with 19 haplotypes, and revealed no obvious phylogeographic pattern. The pairwise F_ST values between the populations varied from -0.049-0.666. Generally, little genetic differentiation (F_ST = 0.108, P < 0.001) among different geographic populations was detected, with the only significant differentiation between the Lucheng (LUC) and other S. exigua populations. The analysis of molecular variance (AMOVA) showed that the percentage of variation within a population (89.18%) was greater than that between the populations (10.82%). The results of neutrality tests on the S. exigua data set of the total population are: Tajima's D (D = -1.897, P < 0.05) and Fu's Fs (Fs = -4.424, P < 0.05), combined with the unimodal mismatch distribution, indicated recent population expansion of S. exigua in large spatial scales.