A clock source determining method and an apparatus are provided. The method includes: A session management network element receives a session request from a mobility management network element. The session request includes information about a network slice and a DNN, and the session request is used to request to create a session. The session management network element determines clock source information corresponding to both the information about the network slice and the DNN. The session management network element sends the clock source information to an access network device. The clock source information is used by the access network device to perform clock synchronization with a terminal device.