Growth and resistance are the most important agricultural traits for genetic breeding in aquaculture animals. The growth trait of Rainbow trout is central to the economic development in scaled production. Launched from strains of Bohai, Denmark, Norway, Donaldson and California, family selection of the rainbow trout was performed for four consecutive generations based on biallele crossing design. Only data from the fourth generation was selected for this study. Dynamic genetic analysis was conducted using 19299 records repeated body weight (BW) and body length (BL) measurements, which were obtained to genetically evaluate the growth traits of 4368 samples for the fourth generation at their 516 days of age, 608 days of age, 668 days of age, 883 days of age and 1036 days of age. According to the Bayesian information criterion (BIC), Legendre polynomials of three orders were selected as the most optimized submodel to fit changes in additive genetic and permanent environmental effects on both BW and BL. With a bivariate random regression model (RRM), both traits were analysed simultaneously. The heritabilities were estimated to exhibit a downward tendency between 400 and 1000 days of age, from 0.288 to 0.164 and from 0.469 to 0.186 for BW and BL, respectively, while BL was inherited in a consistently higher manner than BW. The genetic correlation of BW and BL showed a tendency towards the decrease of heritabilities with the enlarged growth space. However, the traits in the initial and later days of age showed higher correlation for both traits which are all above 0.75, especially for BW, which is higher than 0.85. The genetic correlations in the same growth days of age for both traits are equal to or above 0.75, but decreased from 0.83 to 0.63 in different growth days of age. In summary, the genetic correlation of a single trait or both traits between paired day-ages decreased with the increasing age-interval. However, the consistent genetic correlation between paired day-ages makes the genetic selection for BW feasible at an early stage. These results provide the theoretical basis for breeding selection focused on BW and BL growth traits. At the same time, this research also provides accurate genetic analysis results which is most fitted for the combined selection. Due to the existing higher genetic correlations between early and later stage for BW and BL, the combined selection is suggested from early 400-growth-day. In addition, the big population with couple families will be artificially divided into several individual subpopulations and repeated random measurements for growth points series will be executed for every subpopulation. This technique will satisfy the requirements for an improved fitting with the growth curve and also save money, including from decreased labour costs.