One of the largest contiguous karst areas in the world is southwest China, where special geological conditions, shallow soil layers, the frequency of precipitation extremes, unreasonable land use, and dam construction have rendered the relationship between water discharge and sediment load increasingly complex. Few studies have been devoted to this region to clarify the potential influencing factors of the water–sediment relationship on a monthly basis. Correspondingly, the present study evaluated the temporal variations in sediment rating curves (SRCs: $\mathit{Qs}=a{Q}^b$, where Q_s is the sediment concentration or load, Q is the water discharge, the coefficient a and index b are fitted parameters) and decoupled the effects of climate change and vegetation dynamics on SRC parameters using partial least squares-structural equation modeling (PLS-SEM). To these ends, 2003 to 2017 data on the monthly water discharge, sediment load, climatic and vegetation factors of six karst watersheds were collected and analyzed. No significant trends in interannual variations of SRC parameters occurred. The PLS-SEM showed that the combined effects of climate and vegetation explained 18% to 43% and 8% to 57% of the variations in coefficient a and index b, respectively. In most cases, however, no significant relationship was found among climate, vegetation-related factors, and the SRC parameters. Special hydrogeological conditions and dam construction in the area increasingly complicated the sediment transport process, thus potentially weakening the influence of climate and vegetation on the SRC parameters. This study can serve as guidance for regional ecological management and the sustainable development of soil and water resources.