The scientific extraction of urban built-up area information and the exploration of the spatial-temporal characteristics of urban expansion have significant relevance for urban planning and management. In this study, the Local-Optimal Thresholding method was refined for the quantitative extraction of urban built-up area information within the Baiyangdian basin, and the extraction process utilized PANNDA nighttime light data and Landsat series data from four periods: 1990, 2000, 2010, and 2020. Subsequently, an analysis of the spatial-temporal characteristics of urban built-up area expansion in the basin over the past 30 years was conducted using the urban expansion index and landscape index. The results show: (1) The Optimized Local-Optimal Thresholding method was successfully used to extract the data of the built-up areas in the basin for all phases, and it was confirmed that the method exhibits enhanced applicability compared to its pre-optimization state. (2) The urban built-up area in the Baiyangdian basin experienced significant expansion throughout the study period, with a growth rate of 154.48%. The sizes of the built-up areas across the 35 cities in the basin exhibited high heterogeneity. (3) Temporally, the expansion of built-up areas predominantly exhibited an accelerating trend, with a widening disparity in the pace of expansion among the cities within the basin. Spatially, the built-up area to the left of the Zhengding-Zhuozhou line was less developed than that to the right. Notable expansion trends were observed in municipal districts or county-level cities such as Lianchi District, Jingxiu District, and Gaobeidian City. (4) Based on the results of the landscape index, the expansion of urban built-up areas in Baiyangdian Basin showed the spatial characteristics of ' dispersion-fusion ' during the study period, and the boundary of built-up areas showed the evolution characteristics of 'regular-complex-regular'