Objective Trace elements play an important role in the reconstruction of the paleoredox states of marine sediments. Among them， molybdenum（Mo） has become an ideal proxy for the reconstruction of the redox state of ancient water bodies because it is mainly enriched in the form of autogenetic chemical deposition and less affected by terrigenous debris. However， compared with the ocean， lakes are more susceptible to terrigenous detritus and have more complex sedimentary environment， and the difference in the environment affects the settlement， distribution， and enrichment of Mo. In this study， the surface sediment of Qinghai Lake， the largest lake in China is the main research object. The enrichment mechanism and controlling factors of Mo element in alkaline， suboxic， and brackish water lakes were identified to provide a scientific basis for the applicability of Mo element in the reconstruction of lake paleoenvironment. Methods The planar distribution characteristics of Mo in the surface sediments of Qinghai Lake were determined by high density sampling and systematic testing， including X-ray fluorescence spectrometer （XRF） and inductively coupled plasma-mass spectrometry （ICP-MS）. In addition， the coupling relationship between Mo content and terrigenous input， particle size， element content， total organic carbon （TOC）， and depositional environment was established to determine the main controlling factors of Mo enrichment in the surface sediments of Qinghai Lake. Results The preliminary results show that Qinghai Lake is a brackish water lake with alkaline and weak oxidation. The planar distribution of Mo in the surface sediments of Qinghai Lake increases from the coastal shallow water area to the deep water area in the lake， and Mo is slightly enriched in the deep water area. Under sulfidic conditions， the soluble Mo in the water body is converted to particle reactive thiomolybdates. Because the overall water body of Qinghai Lake is in a state of oxidation， the enrichment of Mo in the sediments is unrelated. The enrichment of Mo in the surface sediments of Qinghai Lake may be controlled by the adsorption effect， that is， it is adsorbed and precipitated by clay minerals， Fe-Mn oxyhydroxides， organic matter， and other substances. The correlation analysis between Mo content and terrigenous input， particle size， element content， and TOC in the selected two transects shows that Mo has a positive correlation with TOC， and the change is similar， whereas Mo has a weak correlation with Al₂O₃ and Zr， which represents the terrigenous detritus input， and the change is significantly different. There was no correlation with clay minerals and Fe-Mn oxyhydroxides. This indicates that the influence of terrigenous detrital input on the distribution of Mo content in Qinghai Lake sediments is weak. Affected by the pH of water and the redox state of surface sediments， clay minerals， Fe-Mn oxyhydroxides have little influence on Mo enrichment in this environment， and TOC is the main controlling factor of Mo enrichment. Conclusions For alkaline and brackish water lakes with weak oxidation of bottom water， such as Qinghai Lake， organic matter adsorption and preservation are the main controlling factors for Mo enrichment in sediments. The type of organic matter could also affect the application effect of Mo as a proxy for redox identification of palaeolakes. Although the type of organic matter may influence the identification of redox conditions by Mo enrichment degree， Mo enrichment in weakly oxidized lake sediments is mainly controlled by the adsorption and preservation of organic matter， and Mo enrichment in sulfidic lake sediments is controlled by both the adsorption and preservation of particles such as organic matter and the reaction with H₂S in the water and the final preservation in the sediment （MoS₂）； thus， the Mo enrichment capacity is stronger. Therefore， Mo is an effective proxy for judging the redox state of palaeowater in the lake basin.