It has been found that the fatigue life of tubular joints is not only determined by the hot spot stress, but also by the stress distribution through the tube thickness represented as the degree of bending (DoB). Consequently, the DoB value should be determined to improve the accuracy of fatigue assessment for both stress-life curve and fracture mechanics methods. Currently, no DoB parametric formula is available for concrete-filled rectangular hollow section (CFRHS) K-joints, despite their wide use in bridge engineering. Therefore, a robust finite element (FE) analysis was carried out to calculate the DoB of CFRHS K-joints under balanced-axial loading. The FE model was developed and verified against a test result to ensure accuracy. A comprehensive parametric study including 190 models, was conducted to establish the relationships between the DoBs and four specific variables. Based on the numerical results, design equations to predict DoBs for CFRHS K-joints were proposed through multiple regression analysis. A reduction of 37.17% was discovered in the DoB, resulting in a decrease of 66.85% in the fatigue life. Inclusively, the CFRHS K-joints with same hot spot stresses, may have completely different fatigue lives due to the different DoBs.