Mengdong Group complex, located in the Laojunshan Song Chay metamorphic core complex between WenshanMalipo strike-slip fault and Red-River fault zone, is one set of volcanic-sedimentary-metamorphic complex undergoing several deformation metamorphism episodes. Geochemical characteristics of the metaclastic sedimentary rock show that average contents of the major and trace elements are similar to the upper continental crust (UCC), and the ratios of SiO2/Al2O3 and K2O/Na2O indicate that the metamorphic sedimentary rocks have similar maturity, but schist samples may suffer strong weathering. Rare earth element (REE) characteristics and metamorphic protolith restoration diagrams indicate that protolith of the schist of the Mengdong Group complex is clay and argillaceous and the protolith of gneiss is sandstone and greywacke. The chemical alteration index (CIA), composition variation index value（ICV） and plagioclase alteration index (PIA) suggest that the source area of the schist suffered a strong chemical weathering process and the source area of the gneiss suffered a relatively weak chemical weathering process. The ratios of Rb/Sr and Th/U indicate that the set of clastic rock has experienced complicated sedimentary evolution process. La/Th-Hf diagram and sandstonemudstone suites discrimination plot show that the sediments of metaclastics were sourced mainly from quartzose sedimentary provenance with minor amount of felsic and intermediate igneous rocks. Diagrams of Th-Sc-Zr/10 and La-Th Sc indicate that metaclastics sedimentary rocks of Mengdong Group complex may deposit in the continental island arc environment. Based on the above analyses, the metamorphic sedimentary protolith of Mengdong Group complex is a set of argillaceous rocks and sandstone (greywacke), which experienced strong weathering and complicated sedimentary evolution process, and mainly deposited in the continental island arc environment. It can be concluded that the protolith of Mengdong Group complex may have had a source from Kunyang Group and has an affinity to the Yangtze block.