In order to clarify the spatiotemporal differentiation characteristics of water conservancy modernization, an evaluation index system for water conservancy modernization was constructed, including four major goals of demand guarantee for resource, environment, ecology, and security, and 14 criteria. A comprehensive evaluation method for water conservancy modernization was proposed, integrating the goal consistency method, coefficient of variation method, and hierarchical weighting method. The level of water conservancy modernization in China and its provinces (autonomous regions and municipalities) was evaluated, and the spatiotemporal differentiation characteristics were analyzed. The evaluation results show that China’s water conservancy modernization has gone through an initiation period, an acceleration period, and a speed-up period, with a generally high level of achievement. The levels of achievement in resource and security demand guarantees are high, but those for environmental and ecological demand guarantees are relatively low. Provinces (autonomous regions and municipalities) are accelerating the process of water conservancy modernization, with the eastern and central regions progressing significantly faster than the western and northeastern regions. Resource and security demand guarantees have been well achieved, while environmental demand guarantee lags behind, and ecological demand guarantee is poorly met. At present, provinces (autonomous regions and municipalities) in China urgently need to accelerate the construction of water ecological environment, improve the guarantee levels for environmental and ecological demands, and accelerate the promotion of high-quality development in water conservancy to advance the process of water conservancy modernization in China.