Abstract: In order to improve the accuracy of modal identification for offshore platforms under irregular waves, a method to find the peak and trough frequencies of wave resultant force aiming at 4-leg offshore platforms is proposed by analyzing the mechanism that the distance between legs along the wave propagation direction affects the frequency property of wave resultant force. Furthermore, on the basis of it, a method to choose the appropriate range of wave striking angles for avoiding the disturbance of colored noise induced by wave resultant force on modal identification is proposed corresponding to each lower-frequency mode. Then by employing a typical 4-leg jacket-type offshore platform, numerical simulations are adopted to validate the method of finding the peak and trough frequencies of wave resultant force, to analyze the disturbance on modal identification due to the peak and trough frequencies of the wave resultant force, and to choose the appropriate range of wave striking angles corresponding to three lower-frequency modes respectively.