The invention relates to a casing drag-reduction device. The casing drag-reduction device comprises a hollow housing, an outer cylinder and a core body; a groove is formed in the inner surface of the housing; the outer cylinder is arranged inside the housing; the side wall of the outer cylinder is provided with a through hole which is matched with the groove; the outer cylinder penetrates the through hole to extend to a pressure-bearing sliding block arranged in the groove to be connected with the housing; the core body is arranged inside the outer cylinder and can abut against the pressure-bearing sliding block; the core body comprises an opened upper core and an opened lower core; the opened upper core is connected with the outer cylinder through a shear pin; the opened lower core is fixedly connected with the downstream portion of the opened upper core; the bottom of the opened lower core is provided with a liquid channel which is communicated with an inner chamber of the opened upper core; an opening of the liquid channel is formed at the bottom of the opened lower core. According to the casing drag-reduction device, air columns can be formed in a downstream space of the casing drag-reduction device and accordingly the buoyancy of pipe strings in the liquid such as the well drilling liquid can be increased and the friction resistance between the pipe strings and a borehole wall can be reduced so as to enable the pipe strings to enter into a well smoothly.