A link down detector and a link down detecting method for Ethernet are provided. The link down detecting method includes the following steps. Firstly, a received signal is received, and a high-frequency band signal is extracted from the received signal. Consequently, the high-frequency band signal is formed as an extraction signal. Then, a high-frequency band power value of the extraction signal is calculated, and a full band power value of the received signal is calculated. Then, a ratio value of the high-frequency band power value to the full band power value is calculated. In a link up status, if the ratio value is changed dramatically in a specified time, a link down signal is asserted to indicate that a network device connected to the Ethernet is switched to a link down status.