A session establishment method and apparatus are provided. The method includes: determining, by a first network device, whether an air interface resource of a default flow of a PDU session is successfully established; and if the air interface resource of the default flow of the PDU session fails to be established, the first network device, based on first information, establishes the PDU session in an inactive state or triggers to release the PDU session, where the first information is used to indicate whether establishment of the PDU session in the inactive state is allowed when the air interface resource of the default flow of the PDU session fails to be established.