By comparison the determination methods of wave under-keel clearance for designing channel in different countries,we analyze the main factors of affecting the under-keel clearance,discuss the application range of the different methods with global wave factors distribution,and compare the influences of different wave factors by a specific example.The results show that the under-keel clearance is the function of wave height,wave direction and wave period in most methods.The under-keel clearance is proportional to the wave height,and the most unfavorable wave direction is 90°or around 45°.The under-keel clearance is increased with the wave period，and there is a maximum when the ship natural period is close to the encounter period.