The oriental armyworm Mythimna separata (Walker) (Lepidoptera: Noctuidae) is a major migratory pest of cereal crops in East Asia, South Asia and Australia. To comprehensively understand the ecological tolerance of M. separata, we collected life table data of individuals from four consecutive generations reared under outdoor natural fluctuating temperatures from 15 April to 17 October 2018 in Yangling, Shaanxi, China. The results showed that the immature stage in early summer and summer were shorter than in spring and autumn. High mortality in late larval instar and pupal stages was observed in the summer generation. The adult pre-oviposition period in autumn was longer than the other seasons. The population in the earlier two seasons had heavier pupae and higher fecundity than the population in the latter two seasons. The intrinsic rate of increase and the finite rate of increase was the highest in early summer (r = 0.1292 day−1, λ = 1.1391 day−1), followed by spring (r = 0.1102 day−1, λ = 1.1165 day−1), and was the lowest in summer (r = 0.0281 day−1, λ = 1.0293 day−1). The results of this study would be useful to predict the population dynamics of M. separata and deepen our standing of the adaptiveness of this migratory pest in natural fluctuating ambient environments.