The present invention is directed to a universal serial bus (USB) transaction translator and an associated IN/OUT bulk transaction method. A device interface is coupled to a device via a device bus, and a host interface is coupled to a host via a host bus, wherein the host USB version is higher than the device USB version. At least two buffers configured to store data are disposed between the device interface and the host interface. A controller stores the data in the buffers alternately. In a bulk-IN transaction, before the host sends an IN packet, the controller pre-fetches data and stores the data in the buffers until all the buffers are full or a requested data length has been achieved; the pre-fetched data are then sent to the host after the host sends the IN packet. In a bulk-OUT transaction, the controller stores the data sent from the host in the buffers, and the data are then post-written to the device.