With the development of the times and users' demand for intelligent and connected vehicles, traditional in-vehicle networks can no longer meet these needs. More and more people are turning their attention to high-bandwidth, low-cost, and high-scalability automotive Ethernet, and it is particularly important to develop and test automotive Ethernet. This paper studies and analyzes the MDI interface of automotive Ethernet, and explains its composition and working principle. After the part selection analysis, the test verification is carried out, and the results show that the test sample passes the return loss test.