In order to study the mechanism of cutting fluid penetration during minimum quantity lubrication (MQL) machining, the flow field in cutting area was investigated through both theoretical analysis and computational fluid dynamics (CFD) simulation on the basis of cutting experiments. Combining the capillary model of cutting fluid penetration, the flow field in the wedge-shaped area beside major flank of the tool was analyzed in detail. It was found that the flow pressure at the wedge angle was lower than the outer. It was favorable for cutting fluid to penetrate into the main cutting edge. Besides, the air supply pressure of MQL has great impact on cutting fluid droplet track. Proper air supply pressure was in favor of cutting fluid penetration.