The Microwave Imager Combined Active/Passive (MICAP) is a suit of active/passive instrument package, which has been proposed for demonstrating the capability of remote sensing the sea surface salinity (SSS), sea surface temperature (SST) and wind speed (WS). In this paper, a nonlinear optimization algorithm for simultaneous retrieval of the above parameters is described. The sensitivity of active/passive microwave observations to SSS is analyzed using the nonlinear optimization algorithm. The results show that the root mean square (RMS) error on the retrieved SSS estimated by using the nonlinear optimization algorithm is enough to meet the requirement of Ocean Salinity Satellite. This is submitted for the special session of “New Developments of Chinese Oceanographic and Meteorological Satellites”.