To trigger the gas switch with one fiber, a kind of LD (laser diode) triggered three--electrode gas switch based on photoconductive semiconductor without trigger charger source is presented in this paper. The principle of this switch is that a GaAs PCSS, which is triggered by LD, is used to control the trigger pulse generation of the trigger capacitor discharge circuit (CDC). The capacitor in CDC is charged with the energy of the main circuit directly. As a result, this switch can work without trigger charge power source, which means this switch can be made more compact compared with traditional electrical trigger switch. A prototype designed for three-electrode gas switch working under 40 kV/8 kA and PCSS triggered with 83 μJ laser energy is described in this paper in detail. The experimental result proves the feasibility of the switch. The experimental result also shows the life of the photoconductive semiconductor is just several hundred shots. The life of the photoconductive semiconductor will be improved in next phase of our research.