Thymallus arcticus belongs to the Salmonidae family and is a valuable cold-water fish, mainly distributed in the Irtysh River Basin. However, overfishing and habitat destruction have depleted their populations and in 2021, they were classified as national level II protected wild animals. Therefore, basic research is urgently needed to better understand the biological characteristics of T. arcticus. To determine the relationship between the morphological traits and body weight of T. arcticus, individuals with ages 1⁺ , 2⁺ , and 3⁺ were used as the research objects and measured for their body weights (Y) and nine morphological traits, namely body thickness (X₁), eye interval (X₂), body length (X₃), body height (X₄), head length (X₅), eye diameter (X₆), proboscis length (X₇), caudal peduncle length (X₈), and caudal peduncle height (X₉). Relevant analysis, path analysis, and multiple regression analysis were used to screen out the main morphological traits affecting the body weight of T. arcticus and establish regression equations. The results showed that: (1) different types of morphological traits were significantly related (P＜0.05) to the weight of T. arcticus at different ages and the number of traits also decreased as age increased. (2) Path analysis retained 4, 4 and 2 morphological traits for T. arcticus with ages 1⁺, 2⁺, and 3⁺ years, respectively. The body length (X₃) had the greatest direct effect (0.345), while caudal peduncle height (X₉) had the largest indirect effect (0.745) on the body weight of 1⁺ -year-old T. arcticus. Body height (X₄) had the largest direct effect (0.473) and body thickness (X₁) had the largest indirect effect (0.378) in 2⁺ -year-old T. arcticus. Body thickness (X₁) had the largest direct effect (0.635) and caudal peduncle height (X₉) had the largest indirect effect (0.344) in 3+ -year-old T. arcticus. (3) The total determination coefficients of the morphological traits retained by the path analysis, and therefore affecting body weight, were 0.943, 0.778, and 0.997 in T. arcticus with ages 1⁺ , 2⁺ , and 3⁺ , respectively. (4) Through multiple regression analysis, the fitted regression equations of the morphological traits (Xi) and body weight (Y) were found to be Y=?90.510+15.345X₁+3.638X₃+10.473X₄+ 16.884X₉, Y=?142.449+29.023X₁+81.082X₂+27.126X₄?47.376X₇, and Y=?228.922+75.063X1+107.864X9 for T. arcticus with ages 1⁺ , 2⁺ , and 3⁺ , respectively. This study identified the main morphological traits that determine the body weight of T. arcticus at different ages and also established the regression equations, providing fundamental research data for T. arcticus conservation biology, as well as candidate morphological traits for future research on the selection and breeding of T. arcticus.