he floating crane vessel can be refitted to improve its workability in the construction of offshore wind farms. Based on the refitted scheme of ChangDa HaiSheng floating crane vessel, study on the allowable sea states during temporary mooring operation of the refitted crane vessel was conducted. In order to determine the allowable sea states of the refitted crane vessel when temporarily moored, a method for assessing the allowable sea states for offshore operations based on the time-domain simulation and probability statistics is introduced. This method is applied to the temporary mooring operations of the refitted crane vessel. First, an appropriate number of wave seeds is selected and the time-domain coupling analysis method is applied to calculate the motion, mooring tension and touchdown mooring length of the moored floating crane vessel. Based on the time-domain simulations, the maximum values of the responses are extracted. Then, the empirical distribution fitting method is used to evaluate the extreme value response of each sea state under the specified non-exceedance probability. The obtained extreme value responses are compared with the safety criterion to obtain the allowable operation sea state described by the significant wave height and the spectral peak period. The results show that after the pulley block refit, the floating crane vessel can carry out the single-ship operation of the monopile lifting, 20 wave seeds are sufficient, and the obtained allowable sea conditions can provide references for the decision-making of temporary mooring operations of the refitted crane vessel.