The tertiary industry plays an important role in China’s national economy. It is of great significance to identify the relationship between water use and industrial development and reasonably control the development of water use in order to control the growth of total water use in the future. Based on the Tapio decoupling model and the LMDI decomposition model, we analyzed the spatial and temporal evolution of the decoupling between the tertiary industry economy and water use, as well as their driving effects in 31 provincial-level administrative regions in mainland China during 2000—2020. Based on the analysis results, a scenario analysis was carried out on the water use target and water saving pressure of the industry in the future. The results show that from the temporal perspective, the tertiary industry has achieved a stable weak decoupling, with an average annual economic growth rate of 12.2%, which is much higher than the average annual change rate of water use (3.6%), and the scope of strong decoupling has expanded nationwide since 2012. From the spatial perspective, the decoupling status of the tertiary industry varied in different regions, which performed well in the eastern and western regions, with the proportion of strong decoupling achieve good decoupling reaching 70% to 80%. In terms of driving factors, the main negative driving effect of the tertiary industry decoupling was economic scale, and the positive driving effect was water saving capacity. The contribution rate of water-saving capacity effect in eastern and western regions reached 56.8% and 52.6%, respectively, whereas the contribution rate was only 38.3% in the central region, because the differences in the driving effects impacted the decoupling. In order to realize the coordinated and sustainable development of the tertiary industry in the future, water saving pressure is inevitable. The process of economic growth has a cumulative effect of scale and time, and the economy will reach 8.8% in 2050 after years of high speed development. Therefore, it is necessary to pay more attention to the water saving capacity to promote a good decoupling of the tertiary industry in the long-term planning, so as to realize the coordinated and sustainable development of economy.