A circuit for generating a peripheral clock for USB, provided on a USB major structure, comprises an internal oscillator, a receiver, a transmitter, a clock counter, and a clock processor; wherein the internal oscillator generates a clock having a settled frequency; the receiver is connected with the internal oscillator and a system unit, and receives a packet transmitted by the system unit; the transmitter is connected with the internal oscillator and the system unit, and transmits a packet of the USB major structure to the system unit; the clock counter is connected with the receiver and the internal oscillator, and counts a length of the packet received; and the clock processor is connected with the clock counter, the internal oscillator, and the transmitter, and controls and adjusts a length of the packet transmitted by the transmitter according to the length of the packet counted by the clock counter.