The value of Cm, the coefficient of hydrodynamic mass, has an important influence on the calculation of the impact energy of mooring ship in the beam sea. At present, there is a great difference among the values of Cm in current relative domestic and oversea documents, and the coefficient value in domestic water transportation industry standards is smaller, which is a hidden danger of building structures. Compared with the regulations on this value, and analyzing the results by referring to formulae and combining the engineering project, we know that the value of Cm in domestic current standards is obviously less than that appeared in other domestic and foreign codes and research reports. It is suggested that Cm for ships larger than 200 000 DWT should be increased to the value between 1.7 and 2.0.