Minimum quantity lubrication (MQL) has been accepted as a successful semi-dry application because of its satisfactory performance in practical machining operations. However, the oil mist generated from MQL machining process has a direct impact on the ambient air quality, and then does harm to those who permanently work in this environment. Using the weighing method, the oil mist concentration in machining workshop is measured. The effect of milling speed on the oil mist concentration under different MQL system parameters such as oil flow rate, air pressure, spray distance to target and jet temperature, is analyzed. The research results show that, the oil mist concentration PM10 and PM2.5 in machining workshop will increase with the increase of milling speed due to the secondary atomization, which results from the fierce collision between oil mist particles generated from MQL system and cutting tool with high rotation speeds. But the varying patterns of oil mist concentration PM10 and PM2.5 are changing with the increase of milling speed at different MQL system parameters.