Abstract: For the shear analysis of interior joints of a reinforced concrete structure by using a Softened Strut-and-Tie model, the effective joint width can always be taken as the whole column width empirically. However, for the frame joint with a cast-in situ slab, the effective joint width is not equal to each other at the top and bottom of a beam column interface. Additionally, the effective joint width also gets wider with the increasing compression stress and ends at the location of crushing. A Modified Softened Strut-and-Tie (MSST) model suggested a more rational calculation method for effective joint width. The shear strength of existing tested 63 interior joints was calculated by Using the Modified Softened Strut-and-Tie model. Furthermore, the theory results were compared with those of the Softened Strut-and-Tie model and those of the code method. The results indicated that the Modified Softened Strut-and-Tie can more precisely predict the joint shear strength than that of the Softened Strut-and-Tie model; Furthermore, the results also indicated that the shear strength of joint core calculated by an MSST model is quite nearly with that of the code method. Thus, the MSST model can reasonable reveal the failure mechanism of beam-column-slab joints.