Porosity is a major casting defect in alloys with large solidification intervals due to the disordered microstructure and broad mushy zones,which decreases badly the mechanical performance.Hence,finding ways to effectively reduce the porosity,further to optimize microstructure and mechanical per-formance is of great significance.In this regard,the Al-Cu-based alloys with large solidification intervals are continuously processed by coupling the travelling magnetic fields (TMF) with sequential solidifica-tion.Additionally,experiments combined with simulations are utilized to comprehensively analyze the mechanism of TMF on the reduction in porosity,including shrinkage porosity and gas porosity,from dif-ferent perspectives.Current findings determine that downward TMF can effectually optimize together the porosity,microstructure and performance,by inducing the strong long-range directional melt flows,stabilizing the mushy zones,and optimizing the feeding channels and exhaust paths,as well as increas-ing the driving force of degassing process.Eventually,downward TMF can increase the ultimate tensile strength,yield strength,elongation and hardness from 175.2 MPa,87.5 MPa,13.3 ％ and 80.2 kg mm-2 without TMF to 218.6 MPa,109.3 MPa,15.6 ％ and 95.5 kg mm-2,while reduce the total porosity from 0.95 ％ to 0.18 ％.However,Up-TMF exerts negative effects on the optimization of porosity,microstruc-ture and performance due to the opposite strong directional magnetic force and melt flows.Overall,our study provides an effective way to optimize together the porosity,microstructure and mechanical performance,and reveals their relationship,as well as details the relevant mechanisms of TMF on the porosity reduction from different perspectives.