With the influx of a large number of vehicles into the coverage area of the Roadside Unit (RSU), the increase in vehicle density makes it difficult for single vehicle authentication protocol to complete vehicle authentication quickly, resulting in authentication delays. In a low-density vehicle environment, batch authentication is usually inefficient and poses certain security risks. Aiming at the balance between security and efficiency, an adaptive authentication protocol for Internet of Vehicles based on vehicle density is proposed. According to the change of vehicle density near RSUs, batch authentication protocol or single vehicle authentication protocol is adaptively selected to improve authentication efficiency. The results of security and performance analysis show that compared to existing protocols, the proposed protocol either meets more security attributes such as conditional anonymity and resist tampering attacks, or has better performance in terms of computation and communication costs.