High-strength　Al-Zn-Mg-Cu　aluminum　alloys　are　widely　used　in　large-scale　aerospace　parts.　However,　high　crack　sensitivity　limits　their　development　during　additive　manufacturing.　The　mechanism　of　crack　formation　in　Al-Zn-Mg-Cu　alloy　parts　manufactured　by　wire-arc　additive　manufacturing　was　studied　through　a　combination　of　thermal　stress　simulations　and　microstructure　analyses.　Increasing　number　of　deposition　layers　gradually　expanded　the　partial　melting　zone,　leading　to　heat　accumulation.　In　addition,　the　equivalent　stress　at　both　sides　was　considerably　higher　than　that　at　the　center　of　the　parts,　and　the　equivalent　stress　at　the　bottom　was　the　highest,　which　gradually　decreased　with　increasing　distance　from　the　bottom.　However,　in　the　initial　stage　of　the　deposition,　the　area　of　the　partial　melting　zone　in　the　bottom　was　small;　the　crack　sensitivity　was　low　(no　cracks　were　found).　With　further　deposition,　liquation　cracks　occurred　40-60　mm　from　the　bottom　and　were　initiated　at　sufficient　stress　and　temperature.　The　increase　in　the　deposition　height　increased　the　stress　and　crack　propagation　to　form　a　macro　crack.(c)　2021　The　Author(s).　Published　by　Elsevier　B.V.　This　is　an　open　access　article　under　the　CC　BY-NC-ND　license　(http://creativecommons.org/licenses/by-nc-nd/4.0/).