[Objectives]  To understand the effects of sex ratio on the reproduction and population dynamics of Spodoptera exigua, and thereby improve our ability to forecast outbreaks of this pest. [Methods]  Reproductive parameters were studied under different sex ratios (♀︰♂=1︰1; 1︰2; 1︰3) under laboratory conditions (temperature (28±1)℃, RH 70%±5%, photoperiod 16 L︰8 D). [Results]  Sex ratio had an obvious impact on reproductive parameters. Although the oviposition duration of females reduced as the sex ratio declined the hatching rate increased. Although female egg production did not differ significantly among the three different sex ratio groups, the contribution of males to the next generation increased rapidly as the proportion of females increased. Offspring sex ratio and female pupa weight tended to increase with declining parental sex ratio. [Conclusion]  Sex ratio had little influence on the reproductive capacity of individual female adults, but did have a significant influence on the oviposition duration of females and male reproduction. Therefore, short-term forecasts based on the number of male S. exigua lured into traps by sex attractants should also take into account the population sex ratio.