Abstract: In response to the spatiotemporal changes in the construction land of typical mountainous cities in southwestern Karst areas during the urbanization process, Guiyang was taken as the research area, and the land use interpreted from remote sensing images of the research area in 2000, 2010 and 2020 was used as the data source. Comprehensive analysis methods such as expansion speed, expansion intensity index, sector analysis and landscape expansion index were used to analyze the expansion characteristics of construction land in the research area in the past 20 years from the aspects of spatiotemporal, morphology, and pattern. The research results showed that: (1) Guiyang’s construction land expansion speed was fast, and the expansion area of construction land was large. The total expansion area from 2000 to 2020 was 285.66 km², of which, the expansion area from 2000 to 2010 was 58.54 km², and the expansion area from 2010 to 2020 was 227.12 km². The areas with strong expansion intensity were mainly distributed in Nanming District and Huaxi District of Guiyang, with the expansion intensity reaching 3.13% and 0.31%. From 2000 to 2020, the expansion direction of Guiyang was mainly concentrated in the three directions of S, SSW and SW, of which the maximum area was concentrated in the direction of SW, covering 71.66 km², 93.78 km² and 167.56 km² respectively. (2) The overall compactness of construction land in Guiyang was relatively low, and the landscape distribution of construction land was relatively scattered, with fractal dimensions above 1.5, indicating that the contour of urban construction land in Guiyang was relatively broken and irregular. (3) From 2010 to 2020, the expansion mode of construction land in Guiyang had mainly been marginal expansion and enclave expansion, the area of marginal expansion and enclave expansion had increased rapidly.