A power supply circuit, a relay device and a Power over Ethernet system related to the electronic technical field are provided. In the power supply circuit, each rectifier module has its positive output end connected to a positive supply pin, and a negative output end connected to a first end of a switch control module. The positive supply pin is connected to a positive terminal of a relay circuit in the relay device. The switch control module has a second end connected to a negative supply pin, and a control end connected to the positive supply pin. The negative supply pin is connected to a negative terminal of the relay circuit. The switch control module allows conduction between the first end and the second end when the rectifier module outputs a voltage greater than a preset threshold. The power supply circuit can supply power to the relay circuit of the relay device without increasing power wiring and affecting the normal power supply of the powered device by the power sourcing equipment. This improves the application flexibility of the POE technology.