The Middle- Upper Devonian sedimentary assemblages are well exposed in the Niushoushan area at the junction of the Alxa and southern North China blocks. The transitional characteristics of their sedimentary facies and source areas are valuable for understanding the Late Paleozoic tectonic evolution of the Alxa and North China blocks and associated adjacent areas. Based on the field studies, a set of sedimentary assemblages deposited by fluvial, lacustrine and small alluvial fan environments were recognized at the Shixiagou section of the southern margin of Niushoushan area, which includes eight sedimentary subfacies: channel, embankment, river plain, lakeside, shallow lake, inner- fan, mid- fan and out- fan. The Middle Devonian Shixiagou Formation is characterized by the transitional deposition from channel to shallow lacustrine environment. The lower part of the Upper Devonian Laojunshan Formation is mainly composed of relatively coarse- grained detrital depositions, including inner- fan, mid- fan, out- fan and channel sedimentary subfacies. The upper part of the Laojunshan Formation is characterized by transitional deposition from lakeside to shallow lake. The number and thickness of conglomerate layers and gravel diameter of the Laojunshan Formation gradually decrease S- ward. Palaeocurrent data indicate that sedimentary source rocks of the lower and upper parts of the Shixiagou Formation originated mainly from NEE- E and SE source respectively. The sedimentary rocks of the Laojunshan Formation mainly derived from NE source. The published paleocurrent and detrital zircon U- Pb dating data of the Shixiagou and Laojunshan Formations, and regional geology demonstrate that the north of the SSE Alxa block was higher than its south during the Middle- Late Devonian. The associated depositions are composed of a set of nonmarine sedimentary assemblage, indicating water gradually deepening southward. The corresponding sedimentary rocks with a northward source are dominated by the Late Ordovician- Early Silurian, early Neoproterozoic, and Paleoproterozoic rocks.