[Objectives]  To investigate the population dynamics of Mythimna separata (Walker) during its south to north migration in China. [Methods]  In 2014 - 2017, 18, 35, 34 and 31 vertical-pointing searchlight-traps were set up in 18, 28, 27 and 26 counties (cities and districts) of 17, 25, 24, 23 provinces (Municipalities and autonomous regions) in China. The number of moths captured was recorded daily over the whole year and periods of peak abundance were systematically recorded. [Results]  Basic information on the range, phenology and abundance of different generations of armyworm in China was obtained, which provides an important foundation for national regional forecasting of outbreaks of this pest. [Conclusion]  Adults occur in South China, Southwest China, the Middle and Lower Reaches of Yangtze River, the Huanghuai River, North China, Northeast China and Northwest China. There are five generations annually. More armyworms were trapped in the area south of 27ºN than south of 30ºN. No migration between armyworm populations was observed in winter. Xiangshan in Zhejiang Province, Fengxian in Shanghai, Dongtai in Jiangsu Province, Changdao in Shandong Province and Luanxian in Hebei Province are the main migration routes of armyworms year-round. Migration between populations were observed in South China, the middle and lower reaches of the Yangtze River, the Huanghuai River, North China and Northeast China. The second and third generations of the North China and the Northeast China populations exchange migrants. Because Southeast Asian countries may contain source populations of armyworms that migrate to China, comprehensive understanding of the population dynamics of the armyworm in China will require further study and international cooperation.