An exemplary port reducing method is for removing unselected ports of an original S-parameter file and generating an optimized S-parameter file. The method controls a display unit to display a user interface to receive commands from a user in response to user operation; the commands comprise a calling command, a selecting command, and an executing command. The method obtains the original S-parameter file in response to the calling command. Next, the method determines which of the ports of the original S-parameter file are unselected in response to the selecting command, and connects each unselected port to the ground through one terminal impedance. The method then generates an optimized S-parameter file that comprises only the selected ports in response to the executing command.