Mythimna separata (Walker) is an important pest which can cause serious damages to cereal crops.  In the past two decades, several heavy outbreaks have taken place in northern China.  In order to develop a fine-scale method of forecasting outbreaks, population data were collected in northern China using searchlight traps and ground light traps.  A background weather pattern analysis and trajectory analysis were performed via the Weather Research and Forecast (WRF) and FLEXPART models.  Our results showed that heavy migration of first-generation M. separata appeared in northern China in 2013.  In Yanqing District, Beijing, the cumulative number of captured adults in searchlight traps was around 250 000 and the daily maximum for trapped moths was 86 000.  During the peak period, the majority of M. separata moths arrived after 00:00 every night.  The sex ratio (female:male) at each monitoring site was greater than 1 and greatly fluctuated with population dynamics.  During the migration peak, prevailing downdraft winds benefited M. separata moths to land passively.  Trajectory simulation showed that immigrants were from Anhui, Jiangsu and Hubei provinces and most of them could continue to fly into the northeastern regions of China.  These results provide technical support for fine-scale forecasting of the outbreak of M. separata at meso- and micro-scale.