With the increase of the proportion of non-fossil fuels in power generation, coal-fired units play an important role in load cycling of power grid more and more frequently, leading to off-design operation conditions for a long time. In this study, the pollutant emission inventory of a 1000 MW ultra-supercritical coal-fired unit is calculated, and its influence on the environment is discussed. The results show that the Photochemical ozone creation potential and global warming potential are dominant factors which have a contribution of about 82% in the total environmental impact potential. With the decrease of load from BMCR to 40%THA, the total environmental impact potential increases gradually, indicating an increasing influence of the power plant on the environment. This is mainly because of the increase in the amount of coal used in lkWh power, and the denitrification efficiency decreases sharply under low loads. The total environmental impact potential under 40%THA load is about 34% larger than that under BMCR load.