Combined power and water systems，in which high-grade energy is used to produce power in a power plant and low-grade heat is used to run a thermal desalination unit to produce fresh water from saline water，are identified to be energy-，economy- and environment-advantageous over separate power-only and water-only systems.The previous studies usually focus on thermal and economic performance of the systems.This study presents an environmental performance analysis on a combined steam-injected gas turbine (STIG) and thermal seawater desalination (TSD) system.Based on thermoeconomics，a mathematical model is formulated for environmental load allocation between power and water in STIG-TSD.After analyzing the life-cycle environmental impact of the system run by natural gas，environmental vectors for each energy stream as well as produced power and water are obtained，and the environmental load allocation between power and water is calculated.Although the study is based on the STIG-based combined system，the method is also applicable to combined power and water systems based on other dry and wet gas turbines.