The imaginary component of the complex index of refraction denotes the attenuation of E-M waves in amplitude within the procedure of propagation in a medium. The attenuation of E-M waves in sea water is of great importance in microwave remote sensing, because the Snell's law of refraction in air-sea interface, sea surface reflectivity, sea surface emissivity and the estimation of sea surface temperature (SST) from microwave radiometer detections are all related to it. The well-known expressions of the reflectivity were derived in optics, but they cannot be applied directly to microwave remote sensing, since the E-M waves of microwave bands propagating in the sea water are greatly attenuated. In this study, a more appropriate formula of the reflectivity is proposed for the full range of E-M waves with considering the attenuation of E-M waves in the sea water. The proposed formula can be used to obtain the emissivity of sea surface, which is useful to estimate SST in microwave remote sensing. The error analysis indicates further that the proposed formula of the reflectivity is significant for improving ocean microwave remote sensing and increasing the retrieval accuracy of SST.