Abstract: The complex urban traffic environment leads to problems of low information transmission security, slow data rate, and limited ground relay in vehicular communication network. A secure transmission scheme for vehicular communication scheme is proposed with RIS-assisted (reconfigurable intelligent surface-assisted) unmanned aerial vehicle (UAV) relay cooperation. In this scheme, the UAV trajectory optimization problem is designed according to the maximum average transmission rate of the system. And then, the non-convex problem of the UAV trajectory is solved by introducing a sequential convex programming alternating iterative algorithm of relaxed variables, which yields the UAV flight trajectory of the maximizing average transmission rate. Finally, the capacity of the eavesdropping channel can be reduced by properly adjusting the position between the RIS and the eavesdropping user, and the wireless transmission environment can be improved. Numerical analysis shows that the maximum average transmission rate can be obtained by optimizing the trajectory of the UAV. With the assistance of RIS, the proposed secure transmission scheme can effectively improve the transmission rate of the communication system and ensure the safety performance though reasonable parameter settings.