A power sourcing equipment (PSE), a powered device (PD) and a line loss detection method for Power over Ethernet (PoE) are provided. The PD receives a voltage/current information transmitted by the PSE to obtain an output voltage value and an output current value thereof, and the PSE receives an input voltage value returned by the PD, such that the PSE and the PD can both calculate a line power loss therebetween based on the input voltage value of the PD, the output voltage value and the output current value of the PSE; alternatively, the PD obtains the input voltage value through a voltage detection circuit under at least one state point of a power consumption that is known, and calculates, according to the output voltage value of the PSE and a known maximum power consumption of the PD, a maximum line power loss therebetween.