In this work, ultrathin nanosheet g-C3N4@Bi2WO6 core-shell structure is fabricated by a new method of CN precursor in situ coating and low temperature reassembled. The CN small molecules were successfully polymerized to form ultrathin g-C3N4 layer (about 1 nm) on the surface of Bi2WO6 nanosheet under low temperature. And the g-C3N4@Bi2WO6 photocatalyst with 1 nm thickness of shell layers has the highest photocatalytic degradation phenol activity with visible light irradiation which is almost 5.7 times as high as that of bulk g-C3N4 and also 1.9 times compare to Bi2WO6 nanosheet. Simultaneously, phenol degradation activity by using g-C3N4@Bi2WO6 photocatalyst under full spectrum is 3.3 times that of bulk g-C3N4 and 1.5 times that of Bi2WO6 nanosheet. Superoxide radicals (center dot O-2(-)) and hydroxyl radicals (center dot OH) as the main oxidative species proved by electron spin resonance spectroscopy (ESR). The interface catalytic system was found, that is the interface between the Bi2WO6 core and g-C3N4 shell effectively promote the phenol degradation activity revealed through a comprehensive contrast experiment. The establishment of g-C3N4@Bi2WO6 core-shell catalytic system can offer blueprints for the construction of other new interface catalytic system.