Disclosed are a device for reducing sloshing impact of a cargo hold for liquid cargo and a method for reducing the same. According to one embodiment of the present invention, the device for reducing the sloshing impact of the cargo hold for liquid cargo comprises: a rise guide unit which is installed at a proper place in a cargo hold, and includes a buoyant floating object that floats to the surface of the stored liquid; and a sloshing prevention member, which is restrained in the rise guide unit, floats the surface of the liquid by the floating object, and suppresses sloshing of liquid cargo.