This study aims to determine the effects of the environmental hormone butyl benzyl phthalate (BBP) on the genital development of the silkworm Bombyx mori. Larvae were fed with different concentrations of BBP at different developmental stages. The numbers of mature eggs produced and laid were recorded, and the genital development was observed. The body weight, survival rate, and egg production were greatly influenced by BBP concentration. In newly molted fifth larva fed with 1.6 mmol/L BBP, 56.1% and 51.4% of mature eggs were produced and laid, respectively, as compared with the controls. The fertilization rate and hatchability of the silkworm showed similar tendencies. The effects of BBP were greater on males than on females. The effects of BBP on the different life stages of silkworm were arranged as follows: newly molted third larva > newly molted fourth larva > newly molted fifth larva. For newly molted third, fourth, and fifth larvae fed with 0.16 mmol/L BBP, 57.5%, 63.3%, 97.6% of mature eggs were produced, respectively, and 58.7%, 74.1%, 96.0% of mature eggs were laid, respectively, as compared with the controls. No evident effects on the cocoon quality of the following generations were observed. BBP, which can accumulate in the silkworm body, has significant detrimental effects on genital development. The earlier the larvae were fed with BBP and the higher the BBP concentration, the greater the biological effects.