The primary task of a selective breeding program to increase the production of Crassostrea gigas is the development of a new, fast-growing, and highly resistant oyster cultivar. Genetic parameters are the basic elements of such a program, which could better realize the intended program. In this study, a mixed-family approach combined with a partial factorial mating design was applied to obtain the experimental materials. Both genetic parameters and genotype by environment (G × E) interactions were analyzed with a mixed linear model and restricted maximum likelihood (REML) using the software package ASReml-R. The results showed the following. (1) the heritability of seven traits ranged from 0.19 to 0.43 for RS (RuShan) and 0.11 to 0.44 for KTD (KongTongDao). Except for the genetic correlations between shell width (SW) and other traits for KTD were not significant, the other genetic correlations between traits for RS and KTD were all positive and significant, 0.48–0.99 and 0.42–0.98, respectively. All phenotypic correlations between these seven traits for RS and KTD were significant and positive, ranging from 0.14 to 0.84 and 0.14 to 0.80, respectively. There is a high positive genetic correlation between the shell-closing strength (SCS) and adductor muscle weight (AMW), indicating that the selective breeding of SCS can play a synergistic role in the selection of AMW. (2) The significance of the genetic correlations between RS and KTD for wet weight (WW) and SCS were both less than 0.8, which meant that the effect of the G × E interactions should be considered. Therefore, G × E interactions had an important impact on the design of WW and SCS selective breeding. As a result, it could be set up for a site-specific breeding program for WW and SCS.