An apparatus for installing a high and low voltage conversion circuit, a high and low voltage conversion system and a power source are described. The apparatus includes: a first housing made of an insulation material and a second housing made of a metallic material, each being provided with two openings at both ends respectively in same direction; first and second housings are fixed, and are in contact on at least part of surfaces thereof; a first cavity and a second cavity are formed in first and second housings for installing a high-voltage circuit and a low-voltage circuit respectively, a high voltage terminal connected with high-voltage circuit and a low voltage terminal connected with low-voltage circuit are inserted through one opening of first housing and one opening of second housing respectively.