To further promote the photocatalytic efficiency of Bi2WO6, g-C3N4 coupling with Sm doped Bi2WO6 2D/2D Z-scheme heterojunction (CN/Sm-BWO) composites were successfully constructed via a facile solvothermal method. The influences of the contents of g-C3N4 coupling and Sm dopants on the photocatalytic performance of Bi2WO6 were investigated in detail. The results showed that g-C3N4 coupling and Sm2+ doping can apparently improve the photocatalytic performance of Bi2WO6. There are about 90% of Rhodamine B (RhB) in 50 min being photodegraded over the optimal 15CN/0.1Sm-BWO sample, being almost 4.1 times higher than that of pure Bi2WO6. The 15CN/0.1Sm-BWO also exhibited high stability. The largely enhanced photocatalytic performance of 15CN/0.1Sm-BWO was mainly ascribed to the synergistic effect of the Z-scheme heterostructure between g-C3N4 and Bi2WO6, and Sm2+ doping in Bi2WO6, thus retaining the high redox capacity and apparently accelerating the separation of electron-hole pairs.