According to the structural features of three main trusses continuous platetruss composite bridge, two compositebeam methods are presented by constructing the displacement mode of compositebeam elements and deriving the stiffness matrix of compositebeam elements. The characteristic of compositebeam method one is that the bridge slab is taken as the top flange of the chord of main truss and combined with the chord of main truss to form a steelconcrete composite beam; the characteristic of compositebeam method two is that the bridge slab is taken as the top flange of longitudinal and traverse beam and form a steelconcrete composite beam with longitudinal and traverse beams. And as for the conventional platebeam composite method, the slab is characterized by forming bridge slab system with continuous isotropic sheets and longitudinal and traverse beams to bear load. Also, the comparison between theoretical results and experimental ones verifies the effectiveness of the above three methods.