The oriental armyworm, Mythimna separata (Walker, 1865) (Lepidoptera: Noctuidae), is a serious global migratory insect pest of grain crops. Although its migratory biology has been studied for a long history, the factors affecting wingbeat frequency (WBF), which is closely related to the flight activity of the insect, remain unclear. In this study, the WBFs of both cultured and migrating moths were tested under different conditions in the laboratory using a stroboscope. The results indicated that age and mating status significantly influenced WBF. One day old adults had the lowest WBF, and unmated females had a significantly higher WBF than that of mated females. In general, the WBF of males was significantly higher than that of female individuals. The WBF decreased gradually with increasing environmental humidity, and WBF had a significant negative binomial regression relationship with temperature change. The WBF of moths that fed on hydromel was much higher than those of the controls that fed on water or without diet. However, wind speed and air pressure had no significant effects on the moth WBF in the test environments. These findings provide a deeper understanding of factors that affect flight ability in M. separata, which will be helpful for developing a regional migratory monitoring and warning system of the pest, such as identifying target insect species based on the WBF from radar observation.