Generally, a passive Q-switching laser has a wider time jitter. In order to reduce the jitter time and the size of the laser, gain switching principle is researched. According to the principle, the pumping power density is controled by the power which generates a step current. A Q-switched laser pumped with a ring laser diodes according to gain switching principle is developed. A 30 mJ output energy and about 16ns pulse duration of this kind of passive Q-switching laser are obtained. There is only one set of pupming source to be used in the experiment. Gain step pulse is directly obtained by controling the waveform of current, and the factors which affect the laser pulse jitter are reduced by this way. Statistic reports of multi-measurement indicate that the laser pulse jittering time is less than 1 μs comparing with the trigger signal. According to the measured stability of pre-pumping and the transient power of the step pulse, the jitter time Δτd is calculated, which agrees well with the experimental result.