A system and method for securely defensing against a collusion attack under Internet of Vehicles (IoV) are provided. The present disclosure can repair a vulnerability, of a reputation system in the IoV, that the IOC attackers can manipulate a traffic-related message aggregation model (TMAM) by increasing their own reputation scores in an inside-and-outside collusion (IOC) manner. In addition, the present disclosure can detect IOC attacks quickly to improve the security of the IoV; can eliminate suspicious providers recursively and provide a reputation fluctuation association rule, to avoid overload of the TMAM; and can deprive IOC attackers of the opportunity to improve their reputation scores and ensure credible information in the IoV, to ensure the fairness and availability of the TMAM without the interference from the IOC attackers.