A data transmission method, apparatus, and system, a network side device, and a terminal device. The data transmission method is applied to the terminal device and includes: setting the terminal device to be in a first non-activated state or a second non-activated state, where in the first non-activated state, the terminal device does not listen to a paging message, and in the second non-activated state, the terminal device listens to a paging message within a first preset listening time or a first preset quantity of listening times, and does not listen to a paging message when the first listening time or the first quantity of listening times is exceeded, setting the terminal device to be in a connected state, and sending, by the terminal device, the uplink data after updating system information; and receiving downlink data that is buffered by the network side device.