Disclosed is a USB transmission system capable of classifying transmission data with a device so as to allow a host to use the transmission data easily. The USB transmission system includes a USB device and a host. The USB device parses the header of an input packet to find out the type of the input packet, tags the input packet with one of identifications according to the type of the input packet, and then outputs the input packet to the host. The host allocates data storage spaces of a system memory that are associated with the identifications respectively, receives the input packet, and stores the input packet in a first data storage space according to a first identification carried by the input packet, in which the first identification is one of the identifications and the first data storage space is one of the data storage spaces.