A data transmission method, a communications device, and a communications system are provided for wireless communications. A terminal device receives a first system information broadcast by a radio access network device. The first system information includes a non-public network (NPN), identification information list and a first association information. The NPN identification information list includes at least one NPN identification information and determines a second system information is valid when first NPN identification information in the NPN identification information list is identical to second NPN identification information stored in the terminal device, and the first association information is identical to a second association information stored in the terminal device. The second system information can include the second NPN identification information and the second association information.