The present application relates to a data transmission method, a host unit, an extension unit, a base station system, and a readable storage medium. The method is applied to a base station system, the base station system includes: a host unit, a plurality of extension units, and a plurality of remote units; the host unit is communicatively connected to the extension units, and each of the extension units is communicatively connected to at least one of the remote units, which includes: the host unit determines a target extension unit corresponding to the user equipment from the extension units according to uplink signal respectively received from the extension units; the uplink signal sent by each extension unit is used to indicate a quality state of signaling between the remote unit connected to the extension unit and the user equipment; sends instruction information to the target extension unit; the instruction information is used to instruct the target extension unit to send uplink data of the user equipment received from the remote unit to the host unit; and receives the uplink data sent by the target extension unit. Using this method can reduce the requirement for the front-haul bandwidth.