Design codes of DNV(Det Norske Veritas), ABS(American Bureau of Shipping), and API(American Petroleum Institute) for offshore platform, whose operation experience and design theory are more mature in the world, were collected. Computing formulas of yield strength for individual members and buckling strength for plate and shell were compared. Then the differences of global strength calculation methods in these codes were studied. It can be found that these three codes are similar in yield and buckling strength assessment for individual structural members, but there are great differences in checking of buckling strength for plat panels and cylindrical shells.