The present status of iron and steel industry of China and Yunnan were given in this work. Base on the analysis of iron and steel industry of Yunnan, we choose two steel plants (marked plant A and plant B) as the typical factories to confirm the emission factor of SO2. The emission factor of SO2 was achieved by mass balance method. In steel plant A, the emission factor is 9.684 (kg-SO2/t-steel), after flue gas desulfurization, the factor dropped to 1.476 (kg-SO2/t-steel). It indicates that desulfuration of sinter flue gas is important to reduce SO2 emission. In steel plant B, the emission factor is 5.4(kg-SO2/t-steel) without desulfurization. However, according to Discharge Coefficients of Industrial Pollutants in the First National General Survey of Pollution Sources(Survey Handbook), the SO2 emission factor of plant B is 4.15(kg-SO2/t-steel). We can see the differences exist between Yunnan’s discharges of pollutants with national Survey Handbook. Because the national Survey Handbook represents the national average level and manufacturing technique and raw materials of various factories vary greatly within the country. So it’s necessary to confirm the emission factor for Yunnan province.