Based on the thermal infrared data obtained from environmental mitigation satellite, the sea surface temperature (SST) was retrieved using generalized single-channel method to get the quantitative information about the diffusion of the thermal discharge around the Tianwan Nuclear Power Plant. It indicated that the retrieved SST was consistent with the MODIS SST product. Based on the retrieved SST distribution map, the thermal field variations in different seasons and tides were analyzed. The area where the SST has increased more than 3℃ in winter was much larger than that in summer. During the winter months, the thermal discharge extended to a sector area around the power station, while in summer it was limited to a longer and narrower area in one side of the bay. The thermal fields at low tide were rather larger than that at high tide, with the area where the SST has increased more than 3℃ four times larger than that at high tide.