Mixed rocks were developed in the Shizigou Formation of Nanyishan Structure surface. As a special type of the sedimentary rocks, its research has some practical significance and scientific value. Based on field observation and microscopic identification, this paper analyzed petrological characteristics of the mixed rock in Nanyishan using X- ray diffraction, major and trace elements, carbon and oxygen isotope test of mixed rock. Considering the complexity of compositions and genesis, this paper employed the effective indicators to determine the sedimentary environment plus excluding a variety of interference factors through data analysis. The results show that the average contents of terrigenous clastics and carbonate minerals in the mixed rocks are 58.7% and 31.0%, respectively, implying that the mixed rocks belong to carbonatic terrigenous clastic rock. Among them, the terrigenous clastic rock is dominantly calcareous mudstones, with detrital minerals consisting mainly of fine silt grade quartz; the carbonate minerals are mainly micritic calcite and clay minerals assemblages are illite and ordered illite/smectite mixed layer; and above three rokcs are mixed homogeneously. Data analysis shows that elements (such as Ca, Na, Mn, Sr, Ba et al.) mainly derived from authigenic minerals. REE, carbon and oxygen isotopic compositions, which were not affected by diagenesis, can be used to determine the salinity and redox conditions of waters during deposition. Combined with assemblage of clay minerals, crystallinity of illite, and chemical index, Sr/Cu and K2O/Al2O3 ratios suggest a cold and arid climate for the depositional period of the mixed rocks; Sr/Ba ratio and Z value indicate a salt water environment; and U/Th ratio, authigenic U and Ce negative anomaly synthetically show a weak oxidizing environment. The results above show that the mixed rocks in Nanyishan are dominantly calcareous mudstones consisting of terrigenous clastic and carbonate minerals, which formed in weak oxidizing saline lacustrine basin under the conditions of cold and dry climate.