The method with high tidal level and tidal range being enlarged by the same design frequency is widely used in the derivation of design tide process at present. By using G -H Copula function, a two -dimensional joint distribution of annual maximum tidal level and its corresponding tidal range was built. Thereafter, the risk probability of the combination of design high tidal level and design tidal range was studied. Long-term observations on Tianjin Port were analyzed and the results indicate that the probability is low when extreme high tidal level encountering high tidal range, the combination risk probability of high tidal level with 50-year return period and tidal range with 50-year return period is 0.05%, the results calculated through enlarging the elements by the same frequency were performed safer, the design criteria of tidal range can be reduced properly according to the risk probability. The joint distribution model and its application provide a new method for deriving design tide process on the basis of quantitative analysis.