A data transmission method includes: A first access network device receives a first paging message from a core network device, where the first paging message includes a first security parameter. The first access network device sends a second paging message to a terminal device, where the second paging message includes a first random access preamble index that is ciphered and/or integrity protected by using the first security parameter. The first access network device receives a first random access preamble from the terminal device, and if the first random access preamble is a random access preamble indicated by the first random access preamble index, obtains downlink data of the terminal device. The first access network device sends the downlink data to the terminal device.