Rack phase difference may be produced in the process of positioning of truss-type Jack-up unit， which will cause a significant additional bending moment at the contact position between the legs and the main hull of the unit， resulting in buckling failure of the leg support pipe at the low guiding position of the hull. In the paper, a jack-up unit is taken as the research object， ANSYS software and birth-death element technology is used to forecast RPD value of jack-up unit in the process of lifting hull by 3-D time-domain numerical simulation analysis， which is divided into four main stages according to changes of RPD value. On this basis， the RPD variation characteristics under three wind speed conditions are analyzed， which is helpful to determine the impact of wind speed on RPD in the process of lifting hull. Finally， a set of RPD control method is proposed based on the law of the change of RPD and the stress of the leg in order to monitor RPD of platform leg.