The invention provides a cable impedance measuring device and method and a network device. The problems that the existing cable impedance measuring method is carried out when the first end of a cable to be measured is impended, and measuring cannot be carried out when an opposite terminal device does not work, cannot start a transmit-receive state or is not connected are solved. The device comprises a signal processing module which is used for outputting a negative pulse signal and determining the cable impedance according to voltage difference between the voltage value of a negative pulse signal and a voltage value characterized by a digital signal and correspondence between different connection states of the first end of the cable to be measured and the cable impedance, a measuring module which is used for carrying out subtraction on the voltage value of the negative pulse signal and the voltage value of a measurement signal to output an analog equivalent voltage signal, and a feedback module which is used for converting the analog equivalent voltage signal into a digital signal and transmitting the digital signal to a signal processing module. When the device is used to measure the cable impedance, regardless of the connection state of the opposite terminal, the cable impedance can be measured.