A vehicle diagnostic system, method, and apparatus are provided, to address insecure effect caused by using a long-term key for a vehicle diagnostic. The vehicle diagnostic system includes a key management system and a to-be-diagnosed unit. The key management system receives a key authorization request sent by a diagnostic device, generates a temporary key based on the key authorization request, and sends a key authorization response to the diagnostic device. The key authorization response carries the temporary key. The key management system configures the temporary key for the to-be-diagnosed unit, so that the diagnostic device and the to-be-diagnosed unit complete a diagnostic based on the temporary key and obtain a diagnostic result. The temporary key is independent of a long-term key in a vehicle. The temporary key is configured to complete the vehicle diagnostic, so that the diagnostic device can be prevented from accessing the long-term key in the vehicle as much as possible. This not only helps protect privacy data of a vehicle owner, but also reduces a possibility that an unauthorized person uses the long-term key to control vehicle running, thereby improving driving security of the vehicle owner.