This study aims to investigate the effect of chemical composition of pyrite with different grain sizes and crystal forms on the oxidation rate of pyrite under natural conditions. In this paper, LA- ICP- MS in situ major and trace element analysis and mineral surface scanning analysis of pyrite in the Bada copper- gold deposit were carried out. The test results show that the coarse- grained pyrite has higher S and Fe content and purer composition, and the trace elements As, Co, Ni and Pb, Cu, and Zn exist preferentially in the fine- grained pyrite in the form of isomorphism and inclusions, respectively. In granular pyrite, both of them can accelerate the oxidation rate of fine- grained pyrite, the content of Cr and Ti in coarse- grained pyrite is higher, and a dense oxide film is formed after oxidation to inhibit further oxidation of pyrite. This paper proposes that different treatment measures should be adopted for the oxidation pollution of coarse- grained pyrite and fine- grained pyrite in most deposits. Coarse- grained pyrite is not easily oxidized, and it can be prevented from oxidation by keeping it in a dry and dark environment at normal temperature and pressure. Fine- grained pyrite is easily oxidized, and the pollution caused by the oxidation products has a great impact on the environment. It should be treated with a combination of source and end treatment for effectively preventing oxidation.