Taking Kunming Lake (test section) in Xi′an as the research object, a two-dimensional hydrodynamic-staining agent coupling model was established based on MIKE21 to quantitatively analyze the water exchange capacity. The results showed that the flow field of Kunming Lake (test section) often forms circulations of different sizes due to the influence of artificial islands and lakeshore topography. The flow rate of the lake area is usually between 0.004 m/s and 0.09 m/s, and the relative larger flow rates often occur near the artificial islands and lakeshores. The hydraulic retention time is roughly stepping up along the main direction of the water flow, and the lake is divided into four different areas. The hydraulic retention time in the western lake area is generally longer than that in the eastern lake area, and the water exchange capacity of the whole lake area coexists, the hydraulic retention time of about 10% of the area does not exceed 10 days, 40% of the area does not exceed 17 days, 70% of the area does not exceed 34 days, and less than 10% of the area is longer than 35 days. Improving the water exchange capacity of the western lake area to avoid water quality deterioration is the key to ensuring the healthy and sustainable operation of the water environment system of Kunming Lake (test section).