Premodern water level observations are unique and effective sources of lake evolution research. Lake Nansi is a large shallow freshwater lake in East China, and its water level observation is among the earliest observations of reservoir and lake water levels in the world. It is composed of four connected lakes: Lakes Weishan, Zhaoyang, Nanyang, and Dushan. On the basis of premodern water level observations and historical documentary records, we reconstruct the water level changes of Lake Nansi on annual and decadal scales over the past 300 years and discuss their relationship with precipitation and the flooding of the Yellow River. A united lake, Lake Weishan–Zhaoyang–Nanyang, appeared in the late seventeenth century, and since then, the lake expanded northward in the following decades. The north boundary extended approximately 3 km during 1684–1755. It expanded significantly further north in 1851 and merged with Lake Dushan, thus forming the united Lake Nansi in 1871. Water level observations of Lake Nansi began in the middle eighteenth century, and regular monthly observations began in 1814, as a result of the drying up of Lake Nansi during 1810–1814 due to a prolonged drought. The purpose of water level observations of Lake Nansi was practical, i.e., to regulate the water supply and ensure the continuous transportation of goods along the Grand Canal. Lake Nansi was flooded by the Yellow River for seven times during 1758–1902. The flooding always resulted in very high water levels, including two prolonged extremely high water level events. The 1851–1855 event marked the highest water level of Lake Nansi, which resulted in a massive northward expansion of the lake. The 1871–1873 event resulted in the formation of the united Lake Nansi. The correlation between the water level of Lake Nansi and a dryness/wetness index in its drainage basin is significant, indicating that precipitation is a dominant factor regulating the water level. The correlation of water level changes between Lake Nansi and Lake Hongze is also significant. Overall, the water level observations of Lake Nansi have proven to be a very valuable archive of regional hydrology and precipitation changes.