Abstract: Conversion of old semi-submersible platform can solve redundant drilling platforms and reduce engineering cost, is new focus of offshore engineering. This paper uses “Nanhai Ⅵ” as target platform, choses an oil field in South China Sea. The platform is converted into a production platform and stability of the platform is analyzed by MOSES software. The main parameters’ influence on stability and balance of the platform is analyzed and the results can give reference to rapid assessment of conversion of semi-submersible platform.