The power supply of flash lamp-pumped a Nd: YAG solid-state laser basically consists of a trigger circuit which initiate the ionization of the gas into the lamp, a simmer circuit that keeps the ionized gas with a low-level dc current and a main discharge circuit where a storage capacitor is discharged into a flash lamp to pump the Nd:YAG laser. The impedance of the flash lamp presents a nonlinear variation and the gas deionization which can occur between two pulses laser, leading to another manual triggering operation. To overcome this operation, we give in this paper a new automated trigger circuit activated by a delivered signal from the simmer circuit. This circuit is simple for implementation and need just few components. It has been simulated by PSpice software and validated by experimental results.