Power-water cogeneration is an effective mode to address fresh water shortage problem, namely low-temperature multi-effect seawater desalination (LT-MED) system coupled with thermal power plant. In the present study, the steam extraction position of thermal vapor compressor (TVC) under different loads was optimized to reduce the cost of water production. And then, the thermodynamic performance of the coupled system is obtained when the extraction position of motive steam varies from No.5 to No.7, whereas the power plant load is in the range of 30% THA – 50% THA. The results indicate that the optimal extraction position is not fixed under different low loads of the power plant, it depends on the cost calculation methods. For any method used to calculate the steam cost, the optimal steam extraction position is from No.5 extraction when the power plant load is 40%THA. The optimal steam extraction position is from No.6 extraction when the power plant load is 30%THA. And the optimal steam extraction position is from No.7 extraction when the power plant load is 30%THA.