In order to study the operational safety of large box floating structures during sea transportation, a physical experiment of hydrodynamic response of floating structures during reverse towing under normal operating conditions was carried out. From perspective of the evolution law of motion spectrum, the six-degree-of-freedom movement of floating structures under irregular wave action was regarded as the studying object. The motion response of the floating structure under the change of significant wave height, mean wave period and towing speed was discussed. The results show that pitch and heave yield a stationary response to the irregular wave load, and are sensitive to average wave period and towing speed. With the increase of the mean wave period, the motion spectra of the pitch and heave transition from multi-peak spectrum to single-peak spectrum, with the shape of the spectrum turning regular and narrowing gradually. In addition, the peak frequency is close to that of the wave spectrum, and the relative peak frequencyis close to 1 and increases with the increased mean wave period. With the increase of towing speed, the shape of heave motion spectrum is gradually sharp and steep, and the relative spectral peak as well as the spectrum peak density grows, indicating intensified hydrodynamic response. Last, the maximum possible heave distance and the maximum possible pitch angle increase with the enhanced wave height, and the maximum possible pitch angle decreases with raised towing speed.