Water is a key restricting factor of socioeconomic development and eco-environmental protection in the arid inland river basins of northwest China. Because of ecological problems caused by climate change and the overexploitation of water resources in the Tarim River Basin (e.g., river breakup, drying-up of lakes, and degradation of vegetation cover), a national ecological water conveyance project has been officially implemented in the lower reaches of Tarim River since 2000. So far, the region’s ecological environment has improved steadily, and studies have evaluated the impacts (e.g., changes in groundwater level and vegetation cover) of the ecological water conveyance project. However, there are relatively few systematic studies on the changes in the area of surface water in the lower reaches of Tarim River in the past 20 years. On the basis of the Landsat 5, 7, and 8 images and the annual water conveyance quantity, this paper used the Google Earth Engine, the surface water extraction method, and the multiple statistic methods (e.g., the Mann-Kendall trend test and the Pearson correlation coefficient) to quantitatively analyze the spatiotemporal changes of the surface water and evaluated the impacts of the ecological water conveyance project on the surface water area. The main results are as follows. Firstly, from 2000 to 2019, a fluctuating upward trend was detected in the overall surface water area, seasonal water area, and permanent water area, with increase rates at 15.54 km²·a^-1, 7.12 km²·a^-1, and 8.41 km²·a^-1, respectively. A fluctuating upward trend has also been detected for all three subregions: the upstream region from Daxihaizi Reservoir to Yingsu station, the upstream region from Yingsu station to Alagan station, and the downstream region from Alagan station to Lake Taitema. Of these, the downstream region had the largest increase rate for both the seasonal water and permanent water areas, with values at 5.23 km²·a^-1 and 8.24 km²·a^-1, respectively. In addition, the increase in surface water area in Lake Taitema was the main factor for the expansion of surface water area in the lower reaches of Tarim River. In 2019, the permanent surface water area, the seasonal water area, and the total surface water area of Lake Taitema were about 267.27 km², 188.00 km², and 455.27 km², respectively; the total surface water area of Lake Taitema in 2019 was evaluated to have been increased by 417.08 km² (about 10.92 times) over that in 2000 (about 38.19 km²). Finally, in the past 20 years, there is a strong positive correlation between the surface water area (especially the permanent water area) and the water quantity of the ecological water conveyance project. The water quantity of the ecological water conveyance project was extremely low during 2007—2009, which directly caused the lowest values of the total surface water area and the seasonal surface water area. Results obtained from this study can contribute to the comprehensive understanding of the impact of ecological water conveyance in the lower reaches of Tarim River and provide a scientific basis for developing regional water ecological protection.