Abstract: Semi-submersible platform is the major equipment in deep sea drilling,and its design and construction techniques are essentially applicable to the environment of the South China Sea.Ultimate strength of the structure represents the limit load it can bear,thus giving full consideration to the uncertainties of ultimate strength has great significance to structure safety.Smith Method was used to calculate the ultimate strength of the semi-submersible platform in this paper,and uncertainties of the ultimate strength were listed with the type of distribution and the characteristic parameters.Monte Carlo Method quantized the uncertainties and the pro bability density curves of the ultimate strength were obtained through cycle calculations.Study results show that the uncertainties of the structure have great impact on the ultimate strength and the nominal value of t he ultimate strength could not truly reflect the bearing capacity of semi-submersible platform.