This study analyzed the spatio-temporal evolutionary characteristics of the Ningxia urban agglomeration along the Yellow River during 1998—2018 from the aspects of morphology, scale, and structure. To this end, the urban built-up area in the study area during the 20 years was extracted from five stages of DMSP/OLS and NPP/VIIRS nighttime light data using a high-resolution data comparison method. The results are as follows: ①The Ningxia urban agglomeration along the Yellow River expanded rapidly during 1998—2018, and the increment and growth rate peaked during 2008—2013. ②The constantly decreased fragmentation degree indicates that the inner morphology of the urban agglomeration became increasingly compact and the built-up patches were relatively concentrated. The fractal dimension continuously decreased, and the geometric shape of the urban agglomeration tended to be regular. Moreover, the urban agglomeration mainly expanded in the means of internal filling. ③The center of the urban agglomeration migrated to the southwest and approached its geometric center, and the urban development accelerated in the south. ④The primacy ratio and Gini coefficient of the urban agglomeration first decreased and then increased, indicating that the gap between cities in the urban agglomeration first narrowed and then widened.