As the connectivity of vehicles increases rapidly, more vehicles have the capability to communicate with each other. Because Vehicular Ad-hoc NETworks (VANETs) have the characteristics of solid mobility and decentralization, traditional security strategies such as authentication, firewall, and access control are difficult to play an influential role. As a soft security method, trust management can ensure the security attributes of VANETs. However, the rapid growth of newly encountered nodes of the trust management system also increases the requirements for trust establishing mechanisms. Without a proper trust establishment mechanism, the trust value of the newly encountered nodes will deviate significantly from its actual performance, and the trust management system will suffer from newcomer attacks. In this article, we propose a trust establishment mechanism based on the Fuzzy Analytic Hierarchy Process (FAHP), which takes into account the historical trust value of the encountered node, the historical driving mileage of the vehicle, the risk values of six factors obtained from the threat analysis and risk assessment (TARA) model STRIDE (Spoofing, Tampering, Repudiation, Information Disclosure, Denial of Service, Elevation of privilege), the road segment trust value of the access point and the interval of separation from the trust management system. Then we use Python to simulate our proposed trust establishment mechanism. It can be seen from the experimental results that the trust establishment mechanism we proposed can make the trust value of the newly accessed node closer to its actual historical performance. The trust establishment mechanism can also improve the identification accuracy of malicious nodes who want to implement newcomer attacks.