Scientific researchers have done a lot of research about the influence of morphological traits of Macrobrachium rosenbergii on body weight of different ages, but the effect of phenotypic traits such as body length before and after sexual maturity on body weight of M. rosenbergii has not been studied. This study aimed to explore the relationship among the phenotypic traits of cultured population M. rosenbergii, before and after sexual maturity. A total of 1 106 samples were collected from a hatchery (Gaoyou), and 15 traits including body weight, total length, and body length were measured, and correlation and path analyses were used to clarify the influence of morphological traits on body weight. The results showed that the key traits that affect body weight and the constructed key phenotypic traits multiple regression equations were extremely between before and after sexual maturity, and between male and female groups. According to the coefficient of determination of individual traits on body weight before sexual maturity of M. rosenbergii, total length, body length, carapace width, carapace depth, abdominal depth and length of second leg should be focused on during selective breeding. Carapace length, carapace depth, the weight of second leg and abdominal weight could be used as key parameters for selective breeding after sexual maturity. Before sexual maturity, body length, carapace width, carapace depth, abdominal length, abdominal depth and length of second leg should be considered during breeding in males while total length, body length, rostrum length, abdominal width, abdominal depth and length of second leg could be considered in females. After sexual maturity, carapace width, carapace depth, abdominal depth, weight of second leg and abdominal weight could be selected in females while considering carapace length, abdominal weight and weight of second leg in males. The present study also indicated that the traits affecting body weight were different between males and females even if in the same growth environment and age, and that males and females had distinct sexual dimorphism. Except for the length of second leg and weight of second leg, in which males were significantly larger than females, and the female’s total length, body length, rostrum length, carapace depth, abdominal length, abdominal depth, and abdominal weight were significantly larger than those of males, which may be related to the fertility enhancement of females. Meanwhile, the multiple correlation coefficient between the retained traits and body weight was higher than 0.85, indicating that the key morphological traits affecting the body weight of each population were verified. In conclusion, the sexual maturity status of M. rosenbergii and the excellent phenotype of males and females could be considered as key factors during breeding practice. This study can provide a reference for the selective breeding of M. rosenbergii.