The invention discloses a method for calculating a dangerous rocking condition of an LNG independent C-type cabin for a ship, and belongs to the field of ship and ocean engineering, and petroleum and natural gas engineering. The calculation method comprises the steps of (1) solving first-order transverse and longitudinal inherent frequencies of the LNG independent C-type cabin under a specified liquid level, and calculating an inherent frequency response range; (2) solving RAO of a real ship based on hydrodynamic analysis; (3) calculating a motion response in a time domain of the real ship according to an operation sea condition, finding out a main frequency, and determining a dangerous rocking excitation frequency in combination with the inherent frequency response range; (4) extracting a motion amplitude value of the real ship in the RAO according to the excitation frequency to serve as a dangerous rocking excitation amplitude value; and (5) solving a rocking load by harmonic excitation of the LNG independent C-type cabin according to the specified liquid level, the dangerous rocking excitation frequency and the amplitude value, and checking structural intensity by the load. The dangerous rocking condition calculated through the method conforms to an actual dynamic response of the real ship and contains a relatively large rocking load of the LNG independent C-type cabin.