An onboard instant messaging method, being implemented by a user terminal, an instant messaging server, a backend system and a head unit, which comprises the following steps: an account association step (S 100) of associating an instant messaging user with a vehicle and storing an association relationship in the backend system; a permission determination step of sending a permission determination request to the backend system from the user terminal via the instant messaging server so that the backend system performs permission determination based on the association relationship, and proceeding to a data exchange step described below if the permission determination indicates success, otherwise ending the process; and the data exchange step of sending messaging data to the backend system from the user terminal via the instant messaging server, and then forwarding same to the head unit by the backend system based on the association relationship, wherein in the account association step, an instant messaging user ID, a vehicle user ID and a vehicle ID are associated and an association relationship is stored in the backend system, wherein the account association step comprises steps S1 to S5, and wherein in step S1, the user terminal sends an association request to the instant messaging server, in step S2, the instant messaging server forwards the association request to the backend system, in step S3, the backend system associates the instant messaging user ID, the vehicle user ID and the vehicle ID and stores an association relationship among the three, in step S4, the backend system feeds back the association relationship to the instant messaging server, in step S5, the instant messaging server feeds back the association relationship to the user terminal. an account association step (S 100) of associating an instant messaging user with a vehicle and storing an association relationship in the backend system; a permission determination step of sending a permission determination request to the backend system from the user terminal via the instant messaging server so that the backend system performs permission determination based on the association relationship, and proceeding to a data exchange step described below if the permission determination indicates success, otherwise ending the process; and the data exchange step of sending messaging data to the backend system from the user terminal via the instant messaging server, and then forwarding same to the head unit by the backend system based on the association relationship, wherein in the account association step, an instant messaging user ID, a vehicle user ID and a vehicle ID are associated and an association relationship is stored in the backend system, wherein the account association step comprises steps S1 to S5, and wherein in step S1, the user terminal sends an association request to the instant messaging server, in step S2, the instant messaging server forwards the association request to the backend system, in step S3, the backend system associates the instant messaging user ID, the vehicle user ID and the vehicle ID and stores an association relationship among the three, in step S4, the backend system feeds back the association relationship to the instant messaging server, in step S5, the instant messaging server feeds back the association relationship to the user terminal.