Abstract: To optimize the cross section of steel box girder of the Xihoumen bridge, a suspension bridgewith amain span of 1 650 m, the aerodynamic performance of twin-box cross sections slotted respectivelywith awidth of5. 5, 6. 0 and 6. 5 m were simulated numericallywith the computational fluid dynamics (CFD) method. The numerical simulation indicates that the aerodynamic performance of the twin-box cross section slotted 6. 0m wide isbes.t The critical flutterspeedsof the twin-box cross section slotted 6. 0m wide, a traditional single box cross section and a twin-box grid cross sectionwere measured by 1∶80 section model tests, and their aerodynamic performances were discussed. The experimental result shows that the three kinds of cross section can meet the requirement for flutter speed of78. 2 m/s, but it is necessary to adopt aerodynamic controlmeasures to a traditional single box cross section