A signal transmission method and related mobile terminal and base station thereof. The method includes: the base station determines a special transmit time interval (TTI) in a downlink carrier (S301); the corresponding time length for which a first type mobile terminal has sent an uplink sounding reference signal to the base station is included in the special TTI; the first type mobile terminal is a mobile terminal which sends the uplink sounding reference signal to the base station in the special TTI; the base station sends a first control signal within a first time length in the special TTI (S302); the first control signal enables a second type mobile terminal to determine that the base station hasn't sent a downlink signal within the remainder time length in the special TTI; the second type mobile terminal is a mobile terminal which does not send the uplink sounding reference signal to the base station in the special TTI. Using the method and the related device enables the future mobile communication system which uses the downlink carrier to introduce the uplink transmission to be compatible with the existing mobile terminals and the future mobile terminals.