In a vehicle mobile device mounting assembly and its auxiliary structure, the auxiliary structure is used with a vehicle mounting kit and mounted together on a rod-like portion of a bicycle. The vehicle mounting kit includes a fastening band. The auxiliary structure includes a fastening body, a joining portion integrally formed with the fastening body. The fastening body includes multiple fastening portions on its outer edges and a fastening space in the fastening body. Multiple fastening openings communicating with the fastening space are formed between the fastening portions. The auxiliary structure is mounted on the rod-like portion of the bicycle by inserting the fastening band of the vehicle mounting kit through a joining portion.