The present invention discloses a synchronizing device for real-time USB audio data transmission, comprising: a first adder unit, a start-of-frame countdown unit, a phase-locked loop circuit, a frequency divider, a second adder unit, a third adder unit, a fourth adder unit, a frame calibrating register unit, a calibrating mapping unit and a calibrating pulse generating unit. A start-of-frame token sent by the USB host is used as a 1-ms reference cycle so that a high-frequency clock passes through the frequency divider to issue a first clock. The first clock signal has a constant clock number in a USB start-of-frame. The absolute time is automatically adjusted according to the duration of a previous start-of-frame. Therefore, asynchronicity of data transmission between the USB host and external devices can be overcome.