Embodiments of this application disclose a data transmission system, including an over the air OTA server, a master electronic control unit ECU, and a node ECU. The OTA server is configured to invoke a first interface of the master ECU. The master ECU is configured to obtain a size of a target software package in response to a fact that the OTA server invokes the first interface. The master ECU is further configured to invoke a second interface of the node ECU. The node ECU is configured to: in response to a fact that the master ECU invokes the second interface, obtain the size of the target software package and check remaining storage space of the node ECU, and when the remaining storage space is not less than the size of the target software package, receive the target software package sent by the OTA server. The solutions provided in this application may be applied to an intelligent vehicle, a connected vehicle, a new energy vehicle, and an autonomous driving vehicle. According to the solutions provided in this application, in a process of implementing in-vehicle software upgrade by using an OTA technology, dependence on a storage capability and a processing capability of the master ECU is reduced, and upgrade efficiency is improved.