With the vigorous development of mobile communication technology and the increasing popularity of Intelligent Connected Vehicles (ICVs), the number of automotive electronic functions and software has grown rapidly. The emergence of Domain Controller Unit (DCU) makes vehicles gradually move towards a domain-centralized architecture. During this period, FOTA (Firmware-OTA, FOTA) technology has been gradually applied to the automotive field due to its ease of use and flexible download. FOTA can help DCU iteratively upgrade, allowing car companies to deploy new in-vehicle software at a faster rate. However, due to the lack of effective Ethernet encryption and authentication mechanisms in the in-vehicle FOTA system, there are still some hidden dangers in information security when the FOTA technology is applied to the DCU software upgrade. In this paper, a new in-vehicle domain controller architecture based on "FOTA DCU" is proposed, and a secure FOTA communication strategy for vehicle controller software oriented to automotive Ethernet is designed according to the national secret SM series algorithm. Finally, a simulation experiment platform of automobile FOTA system is built and tested. The experimental results show that the security upgrade method improves the confidentiality, effectiveness and real-time performance of FOTA upgrade for ICVs to a certain extent.