The invention belongs to the technical field of direct current power supply, and provides a LED lamp and an input polarity automatic converting circuit thereof. An input polarity automatic converting circuit, including a first relay, a second relay, a diode D1 and a switch unit, is employed; when the anode and cathode of a direct current power supply is respectively connected with a negative input end and a positive input end of the input polarity automatic converting circuit, the switch unit is connected through, so as to make the first relay and the second relay are both in a closing state, thus the electric current output from the anode of the direct current power supply normally supplies power to a load via a switch contact of the second relay and a normal-open contact, thereby achieving that the normal power supply to the load can still be ensured in order to make the load work as normal, even if the anode and cathode of the direct current power supply are reversely connected; and the usage of the first relay and the second relay can greatly reduce the loss of electric energy, thus increasing the power supply converting efficiency and the working reliability of the circuit.