A data transmission method and device includes, when the target length of a time-domain resource required by a target transmission block is greater than a first length of an available time-domain resource of a target time slot, the network access device determines a size of a first transmission sub-block transmitted on the available time-domain resource of the target time slot, divides the target transmission block into the first and second transmission sub-blocks according to the size of the first transmission sub-block, in which a second length of a time-domain resource required by the second transmission sub-block is equal to the difference between the target length and the first length, determines a first time-domain resource configured to transmit the second transmission sub-block in the next time slot after the target time slot according to the second length, and sends a first resource allocation message to a terminal.