In order to study the variation law of effective width coefficient of the composite deck of long-span closed composite steel box girder, a finite element model of composite continuous steel box girder bridge is established by relying on G1503 Highway across Wusong River Bridge. The variation law of effective width coefficient of mid-span section and mid-fulcrum section is analyzed under different spans and chamber widths. The difference of effective width between steel deck slab and concrete deck slab is compared, and the suggested value of effective width coefficient of concrete deck slab is given. The results show that the transverse stress distribution of steel deck slab and concrete deck slab is similar. The effective width of the steel slab is basically equal to the specification, the mid-span section is about 0.41% smaller, and the fulcrum section is about 4.13% smaller. The effective width of concrete deck slab is greatly different from the specification, and the mid-span section is about 3.25% small, the fulcrum section is about 27.9% small. The steel deck effective width of the composite bridge deck is greater than the concrete deck effective width, the difference in the mid-span section is 0.51%, and the difference in the fulcrum section is 5.9%. The effective width coefficient of the concrete bridge deck can be reduced by 0.9 times by reference to the effective width coefficient of the steel bridge deck.