According to basin drilling data，logging，core analysis，burial history，paleotempreture，etc.，the spatial distribution，diagenesis and sealing capability evolution mode，porosity，permeability and breakthrough pressures of mudstone in the Upper Paleozoic of south Ordos Basin were studied. And based on that，corresponding evaluation standard and method were proposed to make a comprehensive assessment of the sealing capability of mudstone in the Upper Paleozoic. The obtained results show that：classⅠmudstone caprock in the Shiqianfeng Formation developed with the best sealing capability in the south Ordos Basin，especially in well areas of Zhentan1 and Huangshen1 where the comprehensive evolution weighted value reaches 3.5；abnormal high pressures develop in the Upper Shihezi Formation，where classⅣcaprock distributes widely with the worst physical sealing capability. ClassⅣmudstone caprock is developed in the Lower Shihezi Formation in the west and the south of the research area and classⅠand classⅡcaprock are in the northeast. For mudstone in the Shanxi Formation，those at the late diagenesis C stage is classⅢor classⅣcaprock and those at the late diagenesis B stage is class Ⅰor classⅡcaprock. Industrial gas flow wells distribute in areas with good caprock sealing capability. The comprehensive evaluation results coincide with the exploration practice，which can provide references for gas exploration of the south area.