An apparatus and method for measuring the sloshing in the cargo tank of a liquefied natural gas carrier is provided. The apparatus includes: a plate-shaped floating mat coupler which floats on the surface of the liquefied natural gas contained in the cargo tank of a liquefied natural gas carrier and moves in accordance with the movement of the liquefied natural gas; and an optical sensor measuring acceleration using optical sensor technology to measure acceleration when the floating mat coupler floats by being inserted in one or more positions in the floating mat coupler. The characteristics of the movement occurring in accordance with the sloshing of the liquefied natural gas in the cargo tank of a liquefied natural gas carrier, and the shock and deformation which the inner wall of the cargo tank is subjected to because of such movement, are measured at multiple angles.