The invention provides a semiconductor device and a forming method, a starting circuit and a switching power source of the semiconductor device. The switching power source comprises the starting circuit. The semiconductor device comprises a P-type semiconductor substrate, an N-type drift region, an oxide layer, an intrinsic polycrystalline silicon layer, a doping polycrystalline silicon layer and a metal plug, wherein a source electrode and a drain electrode of a negative threshold field-effect tube are arranged at the two ends of the N-type drift region, the source electrode and the drain electrode are exposed out of the oxide layer, the intrinsic polycrystalline silicon layer is arranged at one end, close to the source electrode, of the oxide layer, and the doping polycrystalline silicon layer is arranged at one end, close to the drain electrode, of the oxide layer. The intrinsic polycrystalline silicon layer and the oxide layer form a grid electrode of the negative threshold field-effect tube, the doping polycrystalline silicon layer forms a resistor connected with the grid electrode, and the metal plug is connected with the drain electrode of the negative threshold field-effect tube and is adjacent to the doping polycrystalline silicon layer. In the semiconductor device, the resistor connected with the drain electrode and the grid electrode of the negative threshold field-effect tube is formed at the position, arranged between the drain electrode and the grid electrode, of the semiconductor substrate and shares the metal plug with the drain electrode, the area of a chip is saved, metal interconnection is reduced through port sharing, and the reliability of the semiconductor device is improved.