This paper presents a comprehensive investigation on local web buckling mechanism and design of double-coped beam connections. Following a careful validation study, a series of finite element (FE) models are established, covering a spectrum of geometric and material variables including cope length, cope depth, web slenderness, and steel grade. The study reveals that the main failure mode of the models is either inelastic or elastic local web buckling, and the considered parameters can evidently influence the buckling capacity. The models with short copes tend to fail by inelastic buckling accompanied by excessive shear yielding. For the models with long copes, especially for those with thin webs and high steel grades, stable post-buckling equilibrium path could be sustained after the occurrence of initial buckling, and as a result the ultimate reaction can be evidently higher than that governed by elastic buckling. In addition, stress concentration is significant near the cope corners, and the peak elastic stress concentration factor (SCF) could achieve around 2.0. A further discussion is made on various support and boundary conditions, and these variables are also shown to have clear influences on the local web buckling capacity of double-coped beams. Based on the numerical results, and recognising the potential limitations of the existing design rule, a modified design method, taking account of the various influential factors revealed in this study, is finally proposed. The available experimental and numerical results show that the modified method can effectively improve the accuracy of local web buckling design for double-coped beam connections.