Influence factors of the breakdown delay jitter of a self-triggered UV-illuminated switch are studied. It indicates that the switch electric field at the injection time of pre-ionization is the decisive factor of the time jitter when the capacitor discharge stage of the trigger gap works. Increasing the operating coefficient and using electrode material with lower working function cannot remarkably reduce the time jitter when the switch breaks down near the peak time. The improvement method is to reduce the value of parallel resistors, which can extend the duration of sufficient arcing of the trigger gap and eliminate the influence of pre-ionization injection time and its jitter. By adapting this method, the time jitter is less than 1.3 ns or 2.8 ns under a pulse rise time of 100 ns or 180 ns when the operating voltage is 300-800 kV.