Bottom blown oxygen copper smelting process developed at Dongying Fangyuan Nonferrous Metals shows many advantages such as flexible feed requirement, high productivity, low energy consumption and high direct copper recovery. However, the weak acid generated during the process is not stable which influences the waste management and continuous operation. The amount of the weak acid is determined by SO₃ formed in copper smelting process. SO₃ can be formed inside copper smelting furnace and boiler at different conditions. Effects of temperature and oxygen partial pressure on the formation of SO₃ during copper smelting process have been discussed thermodynamically. Understanding of the mechanisms for SO₃ formation will enable the operation of bottom blown copper smelting furnace to be optimized and generation of the weak acid in the process can be managed. The fundamentals developed in this study can also be applied to other copper smelting processes.