The present invention discloses a universal serial bus (USB) charging circuit, comprising: a charging path for charging a battery from a USB host; a charging switch located on the charging path; a current sensing circuit for sensing current information on the charging path; a maximum available current detection circuit for detecting the maximum available current from the USB host; and a loop controller circuit for controlling the charging switch so that the charging current on the charging path is substantially equal to the maximum available current detected by the maximum available current detection circuit, wherein the maximum available current detection circuit detects the maximum available current during circuit initialization and stores it.