The operable days forecast of high seas floating dock for supply ship berthing is made. The structural and environmental information of high seas floating dock and supply ship are introduced, and the short-term forecast of structural motion response of high seas floating dock and supply ship is made with the spectral analysis method in frequency domain methods. The forecast of motion response extremum of high seas floating dock and supply ship is made respectively with the time domain method and the spectral analysis method in frequency domain methods, and the two forecast results are compared. According to the forecast results of frequency domain method and the operation constraint conditions, based on the wave operation conditions of three target sea areas with different heading arrangement, the operable days of the high seas floating dock are calculated. The calculation results show that the operable days forecast of high seas floating dock for supply ship berthing with the spectral analysis method can provide reference for the heading optimization and anchorage optimization of high seas floating dock in actual engineering, and provide guidance for the operational capability evaluation of high seas floating dock and other offshore engineering operating ships.