An online calibration method and device for a universal serial bus system is disclosed in the present invention. The method comprises following steps: providing a plurality of chirp JK pairs; detecting the plurality of chirp JK pairs, and loading a power on a terminal resistor of a USB device end of the universal serial bus and its coupled to a terminal resistor of a USB host end of the universal serial bus; detecting a voltage level variation of the chirp JK pair; and processing the online calibration according to the voltage level variation to maintain the voltage level within a preset range.