In the cut-in scenario of the ACC system, there is often a lack of harmony between people and cars due to the limitations of sensors and control strategies. Finding and solving the conflict between the driver and the machine is essential to achieve harmonious Human-Machine Cooperation. This research is to understand the conflict between the driver and ACC system in the cut-in scenario based on the previous work of driver trust experiment. The research selected eight drivers for in-depth interview, and the results showed that the biggest conflict between the driver and ACC was that the driver's cognitive and behavioural patterns were significantly different from the ACC system. It is mainly reflected on three aspects: the different definition of the cut-in scenario, the risk perception and the stress of the impending danger, and the perceptual process of cut-in scenario. In order to reduce human-machine conflict, the research proposed three design strategies: (1) Redefine the cut-in scenario based on the driver's cognition. (2) Keep the ACC human-machine interface consistent with the driver's psychological perception. (3) Help drivers cope with dangerous scenario with three levels of warning signals: guidance information, warning information and takeover information.