In this paper, the probability of a crane ship lifting load system working safely under given conditions is calculated. First, by using Lagrange's equations, we derived the motion equations of the lifting load system, from which the nonlinear responses of the crane ship and the lifting load system under random sea wave load in time domain are achieved. Their maximum are used to calculate the required thresholds of angular displacement according to the given confirmation criteria. By modal analysis, we obtained its random response spectrums, which are analyzed by using probability statistics. The analysis resulted in the variances of the angular displacement and that of the angular velocity. Finally, the variances are used to analyze the dynamic reliability of the lifting load system according to the first-passage failure criterion.