Head-Up display (HUD) is increasingly applied to automobiles. However, HUD might have also shortcomings causing new driving problems. This paper investigates the effects of different Human Machine Interfaces (HMI) of HUD under ACC function on situation awareness (SA) and system usability for cut-in driving scenarios. The laboratory-based controlled experiment conducted used a driving simulator with a total of 8 participants. Each participant performed three different tasks, using three different HMIs including two HUD HMI and one baseline HMI (dashboard HMI). The results indicate that HUD display can influence the participants’ SA and system usability, and that different HUD design can have different effects. The HUD design with dynamic directivity of augmented reality brings about better SA and system usability. The research suggest that it is possible to improve the SA and system usability through improved HUD-HMI design.