This application provides a port detection method, an optical network device, and a passive optical network system, to quickly and accurately detect a port connected to an ONU, and improve efficiency of determining the port connected to the ONU. The method includes: an optical line terminal sends optical signals corresponding to all of N wavelengths to at least one optical network unit, where the N wavelengths are different from each other, and N is a positive integer; the OLT receives optical power values that are of the optical signals corresponding to all of the N wavelengths and that are sent by a first ONU, where the first ONU is any one of the at least one ONU; and the OLT determines, bases on the optical power values of the optical signals corresponding to all of the N wavelengths, information about an optical splitter port corresponding to the first ONU.