<p>The Pacific oyster (<i>Crassostrea gigas</i>) is a key global aquaculture species. Tetraploid oysters are an essential broodstock for sustainable triploid production, yet their growth traits still need improvement. In this study, we successfully constructed 30 full-sib and 10 half-sib families of tetraploid Pacific oysters and reared them at three culture sites: Jiaonan, Rushan, and Rongcheng. We evaluated the genetic parameters of growth traits—shell height and wet weight—and then assessed the effects of genotype×environment interactions (G×E) on these traits. Additionally, the sex and ploidy composition of all tetraploid families was also examined. The heritability was moderate to high for shell height (0.39–0.44) and moderate for wet weight (0.32–0.36). For the same traits, genetic correlations between sites were moderate to high (0.55–0.80), indicating significant G×E effects. Tetraploid families reared in Rongcheng exhibited a strong male bias. Although chromosome loss was detected in all families at 360 d, the proportion of triploids and mosaic tetraploids was low (&lt;10%), which may not affect ploidy. Additionally, the extent of chromosome instability and growth performance did not correlate. This study provides a fundamental genetic basis for breeding tetraploid Pacific oysters. It underscores the importance of considering G×E interactions in breeding programs, while low-level chromosome instability appears not to influence the selection of families for better growth.</p>