High-concentration oil mists can cause serious health problems to workers, which are generally mitigated by ventilation and purification systems. However, the coupling relationship between these systems is not clear. In this study, the effects of purifier outlet direction, purification air volume, installation height, and purification efficiency on indoor oil mist distribution were investigated by numerical simulation using an actual machining factory. The mitigation of oil mist in various combinations of ventilation and purification systems was also discussed. The results show that the outlet direction of the purifier has a great influence on the distribution of oil mist in the factory, and the maximum difference of oil mist concentration in the breathing zone under different orientations is 17%. The best purifier outlet direction is vertically upward. When the purifier outlet direction is upward, a larger purification air volume is beneficial for contaminant removal, and a lower purifier exhaust installation height is helpful for oil mist discharge from the bottom of the factory and reducing the concentration of oil mist in the breathing area. The oil mist concentration of purifier exhaust increases from 0 to 2 mg/m3 and the oil mist concentration in the breathing zone increases by 67%. The combined system of purification system with the roof exhaust system and displacement ventilation system has the optimal pollution removal efficiency and the lowest concentration of oil mist in the breathing zone compared to other systems. The research results can provide a reference for the design, installation, and operation of ventilation and purification systems in machining factories.