The Mengqiguer uranium deposit in the Ili Basin is the largest sandstone-hosted one discovered in recent years. In order to research the source of ore-forming materials, the properties of ore-forming fluids and the metallogenic environment of the deposit, a geochemical studies on trace elements and sulfur isotope of the pyrites (the gangue minerals in Xishanyao Formation and Badaowan Formation, which closely related to ore minerals such as uraninite and coffinite) were carried out systematically. The results show that the REEs of pyrite in each sample exhibits a similar distribution pattern,that is,LREE is significantly more enriched than HREE (La_N/Yb_N=4.27-9.82), with strong negative Eu anomaly (δEu=0.50-0.71) and no Ce anomaly (δCe=0.93-1.04).The Co contents of pyrite range from 2.1×10^-6 to 26.7×10^-6, Ni from 19.5×10^-6 to 79.30×10^-6,the ratio of Co/Ni from 0.07 to 0.88, and As from 8.90×10^-6 to 95.60×10^-6 higher than the average As content of the continental crust. The sulfur isotope composition of pyrite has a relatively wide range of variation, i.e. δ³⁴S_CDT ranging from -17.30‰ to 3.90‰, with the change range of 21.20‰. Combined with characteristics of REEs, trace elements and sulfur isotope composition of pyrites and regional metallogenic setting,it is suggested that pyrite in this deposit is of sedimentary origin and was formed in a low temperature reduction environment. It is concluded that the sulfur of pyrite mainly originates from coal and sedimentary sulfide with microbial genetic characteristics, and pyrites and charcoal debris (organic matters) provide reductants for redox reactions during uranium mineralization.