What is claimed is:1. A voltage conversion device, comprising:a circuit board, comprising a front surface and a back surface, a transformer circuit and an assembling terminal disposed on the front surface of the circuit board, the assembling terminal electrically connected to the transformer circuit, and a through hole disposed on the circuit board at a position corresponding to the assembling terminal; anda metal shielding cover, covering the back surface of the circuit board, and comprising a conductive pin disposed corresponding to the assembling terminal,wherein the assembling terminal comprises a clamp arranged corresponding to the position of the through hole, the conductive pin passes through the through hole and is clamped by the clamp to fix the metal shielding cover on the circuit board.2. The voltage conversion device according to claim 1, further comprising an insulative housing, the circuit board and the metal shielding cover being accommodated in the insulative housing, and a profile of the metal shielding cover being matched with at least a portion of an internal surface of the insulative housing.3. The voltage conversion device according to claim 1, wherein the assembling terminal comprises a base upright arranged on the circuit board, a leg is extended from a bottom of the base and soldered to the circuit board, a pair of claws are extended from a side of the base, and the conductive pin corresponding to the assembling terminal is clamped by the pair of claws.4. The voltage conversion device according to claim 1, wherein the assembling terminal comprises a base, the base is a cylinder upright arranged on the circuit board and surrounding the through hole, a leg is extended from a bottom of the base and soldered to the circuit board, a pair of claws faced to each other are extended from an internal surface of the base, and the conductive pin corresponding to the assembling terminal is inserted in the base and clamped by the pair of claws.5. The voltage conversion device according to claim 1, wherein the assembling terminal comprises a base upright arranged on the circuit board, a leg is extended from a bottom of the base and soldered to the circuit board, a claw is extended from a side of the base, an end of the claw is reversed toward the base, and the conductive pin corresponding to the assembling terminal is clamped between the claw and the base.6. The voltage conversion device according to claim 1, wherein the assembling terminal comprises a base upright arranged on the circuit board, a leg is extended from a bottom of the base and soldered to the circuit board, a claw is extended from a side of the base, and a guiding slope is disposed on the claw, the guiding slope faces the through hole and is obliquely arranged corresponding to the circuit board.7. The voltage conversion device according to claim 1, wherein the metal shielding cover comprises an aluminum or a stainless steel.8. The voltage conversion device according to claim 7, wherein the conductive pin is riveted on the metal shielding cover.9. The voltage conversion device according to claim 1, wherein the transformer circuit comprises a ground area, and the assembling terminal is electrically connected to the ground area.10. The voltage conversion device according to claim 1, wherein another conductive pin is disposed on the metal shielding cover, each of the conductive pins are in a flat shape, and normal directions of each of the conductive pins are non-parallel with each other.11. The voltage conversion device according to claim 1, wherein another conductive pin is disposed on the metal shielding cover and another assembling terminal is disposed on the front surface of the circuit board correspondingly, a force direction of the assembling terminal exerted on the conductive pin is non-parallel with a force direction of the another assembling terminal exerted on the another conductive pin.