Provided is a device for reducing liquid oscillation in a moon pool of an oceanographic engineering structure. Being fixed on the inner wall of the moon pool, a stream guidance cover comprises an outer housing, an inner housing, a plurality of vertical ribs and a plurality of arc-shaped transverse rib plates that are fixedly and integrally connected with the inner wall of the moon pool. The outer housing and the inner housing are both in trumpet shapes. The inner housing is installed between the outer housing and the inner wall of the moon pool. The vertical ribs are radially and uniformly distributed among the outer housing, the inner housing and the inner wall of the moon pool. The plurality of arc-shaped transverse rib plates are fixed at intervals among the outer housing, the inner housing and the inner wall of the moon poll layer by layer. The outer housing and the inner housing are both provided with multiple rows of round holes from top to bottom. The device for reducing the liquid oscillation in the moon pool of the oceanographic engineering structure is simple in structure. The stream guidance cover or multiple layers of wave attenuating rings or multiple rows of arc-shaped wave attenuating plates can effectively prevent the waves climbing, thereby preventing the occurrence of overtopping on the deck, reducing the formation of vortex inside the moon pool, and improving the hydrodynamic property of watercraft. The coupling effect between the sea water and the structure is weakened; and the oscillation of the oceanographic engineering structure is relieved.