Among the various connection forms of tubular joints, tube-gusset joints are widely used in practical engineering due to their simple structure and convenient manufacturing process. When the bearing capacity of the tubular joint is insufficient, filling the chord with concrete has proven to be an effective method of strengthening. However, fewer studies have been conducted on the failure mechanism, force transmission mechanism, and bearing capacity calculation model of the concrete-filled circular steel tubular (CFCST)-gusset joints. In this study, four CFCST-gusset K-joints and one circular hollow section (CHS)-gusset K-joint were experimentally tested, considering parameters such as the chordal diameter-thickness ratio ( γ), the web to chord diameter ratio ( β) and wall thickness ratio ( τ), the gusset plate to web thickness ratio ( t g/ t i). According to experimental investigation, the finite element method (FEM) was employed to analyse the influence of these parameters on the mechanical behaviour of the CFCST-gusset K-joints. The results show that the CFCST-gusset K-joints exhibited two distinct failure modes, namely, web member failure and gusset plate failure. However, the CHS-gusset K-joint failed due to the excessive plastic deformation of the chord tube. The gusset plate to web thickness ratio t g/ t i is the most critical factor affecting the failure patterns and bearing capacity of the CFCST-gusset K-joints. The value of t g/ t i should be larger than 2 in practical engineering to prevent joint failure. A least-square method was used to fit the bearing capacity computational model of the CFCST-gusset K-joints. Regression validation confirmed the high accuracy of this computational model.