A port multiplier system is provided. The port multiplier system comprises a first port multiplier and a second port multiplier. The first port multiplier is configured to receive a plurality of first frame information structures from a host. Each of the first frame information structure corresponds to a first port multiplier port number. The first port multiplier sends the first frame information structures that correspond to the first port multiplier port numbers, respectively, to a first downstream port of the first port multiplier according to first port multiplier port number. The second port multiplier is configured to send the first frame information structures that are sent to the first downstream port to a plurality of second downstream ports of the second port multiplier, respectively. An operation method is also provided.