Multi-box suspension bridges represent an innovative structural typology, characterised by multiple decks that offer greater stability, loading capacity and reduced torsional stresses. Until the construction of the 1915 Çanakkale Bridge, the Xihoumen Bridge (completed in 2009) was the longest multi-box suspension bridge in the world. Its deck’s configuration was studied to provide much better wind resistance performance, considering the stricter stability requirement of 78.4 m/s wind speed. With this structural innovation, the critical flutter speed reaches approximately 90 m/s. The aim of this Thesis is to investigate the flutter instability of the Xihoumen Bridge adopting the finite element software ANSYS APDL and an analytical model implemented in a MATLAB code.