The black cutworm, Agrotis ipsilon (L.) (Hufnagel), is a worldwide pest infesting >30 crops. Previous studies on its migration in East Asia have been mostly carried out in the regions where it could safely overwinter; however, it's still unknown what pattern of seasonal migration this species exhibits across different climate zones (i.e., in north of East Asia). Our study determines combination of searchlight trapping and ovarian dissection on a small island in the center of the Bohai Strait during 2003–2014, providing direct evidence that A. ipsilon undertakes regular migration across the sea every year. The results of ovarian dissection indicated that the ovarian developmental level, the proportion of sexually mature females, mating rate, mating frequency, and sex ratio decreased from spring to autumn. Some degrees of ovarian development and relatively higher proportion of mated females suggest that A. ipsilon is not completely bound by “oogenesis-flight syndrome” and “autumn reproductive diapause.” The results of population dynamics, ovarian development level, and mating rate showed three different types of periods—1 May to 2 July, 14 July to 3 September, and 4 September to 25 October—for the migration of A. ipsilon. Our results may be fruitful for improving the forecasting system of A. ipsilon.