The lifetime test of initial designed nylon packaged coaxial three-electrode gas spark switch has finished. It shows that the lifetimes of three switch samples are 204 000, 433 000 and 151 000 times, respectively. Based on the test results, an improved design with respect to structure and insulator material of switch is proposed. Preliminary test results show that each of the three switch samples with PMMA insulator has finished 300 000 shots, and their self-breakdown voltages keep good stability. According to variation of self-breakdown voltage, the delay time and jitter of switches in triggering experiments, the predicted lifetime of the switch of the improved design will exceed that of the initial former design.