Exogenous 20-hydroxyecdysone (20E) exerts a range of detrimental effects on the development and survival of many insect species, and different species show varying susceptibilities to ingested 20E. The specific effects of exogenous 20E on Plutella xylostella (L.) (diamondback moth; Lepidoptera: Plutellidae), a severe pest of cruciferous crops, has not been reported systematically. Here, we studied the effects of exogenous 20E on feeding, development, and survival of P. xylostella larvae and on fecundity and longevity of adults by using a leaf dip assay. We found that food consumption and the duration of development of larvae that survived to the next instar decreased with the increasing concentrations of dietary 20E. Ingested 20E exerted adverse effects on the development of larvae by decreasing their weight, and led to death mainly by inducing abnormal molting. The lethal effect of 20E on larvae was also determined by a residual film method, which showed LD50 values of 1st to 4th instars were 0.331, 0.345, 0.439, and 0.252 mg/mL, respectively. Female adults laid reduced numbers of eggs on leaves treated with 20E. There was a negative correlation between the concentration of 20E on the leaf surface and the number of eggs deposited on the leaves (P < 0.05). After 5 d, the average fecundity of adult females was reduced and correlated with the concentration of 20E in the diet (P < 0.05). The longevity of male adults was significantly shortened after ingesting diet containing 0.50 mg/mL 20E. Thus, ingestion of exogenous 20E exerted adverse effects on feeding, development, and reproduction of P. xylostella, and 20E residues on leaves of host plant had significant repellent effects on oviposition by females.