Based on the occupational health problems of operators caused by the escape of volatile gas in a rod mill, the emission characteristics of volatile gas in the rod mill cavity were analyzed. By establishing the calculation model of suction and discharge in the inner cavity of the rod mill, the distribution characteristics of the flow field, air age, and volatile gas, as well as discharge efficiency were analyzed. The results show that the gap can reduce the volume fraction of ammonia in the rod mill and improve the discharge efficiency, but the volatile gas will escape from the gap when the pressure difference is less than 15 Pa, so it is necessary to pay close attention to the pressure difference to ensure that the gap does not escape. Under the same working conditions, the average discharge efficiency of the optimization scheme is increased by about 68.7% and 71.8%, respectively, indicating that the sewage discharge efficiency of the optimization scheme is significantly improved. Considering the volume fraction of ammonia, the potential risk of escape, and the discharge efficiency, it is concluded that the pressure difference is the best when the pressure difference is 60 Pa.