All birds reproduce by internal fertilization, but only 3% of birds have external genitalia. Hormone secretions and body size influence genital growth, but the actual regulatory mechanism is rarely reported. Thus, using 35 geese as experimental material, the regulatory mechanism of goose external genitalia growth was explored by measuring body size parameters, serum hormone concentrations, and related gene expression. In this study, genital growth was different among tested geese, but histological and morphological results showed that all geese external genitalia contained complete tissues. Measurements of hormone levels showed that at puberty, as the genital length increased, irregular decreases were observed in the levels of follicle-stimulating hormone (FSH) and luteinizing hormone (LH), whereas an irregular increase was observed in the levels of testosterone (T); furthermore, the levels of testosterone (T) gradually increased to a peak at 34 weeks. Based on RT-PCR results, as the genital length increased, only the expression of 17α-hydroxylase/17, 20-lyase (CYP17) mRNA slightly decreased at first, and then significantly increased to a peak, whereas the expression patterns of other genes were irregular. Furthermore, the CYP17 immunohistochemistry results also showed a pattern that was highly consistent with the patterns of mRNA expression and T secretion. In addition, based on body measurements, as body weight increased, the genital length increased. Thus, these results suggested that the CYP17 gene plays a key role in goose genital growth.