Modern vehicles are becoming increasingly automated, and while their internal CAN network provides connectivity, it also offers opportunities for cyber attacks. Vehicle safety is a fundamental issue in the study of vehicles, and only with sufficient understanding of the impact of attacks can more targeted protective measures be developed to better protect vehicle safety. A subsystem based network attack impact assessment model is proposed to quantitatively evaluate the impact of attacks on the CPS network layer of automobiles. This model measures the impact of attacks on subsystem buses and nodes from two dimensions: availability and integrity. The model was used in the attack experiment to calculate the impact of the attack, verifying its effectiveness and enabling quantitative evaluation of the impact of the attack.