Abstract: In practice, the working properties of the 120 gas switches which play very an important role in the operation of LTD on Qiangguang I accelerator vary with the number of discharges. With the accumulated shots of accelerator, the gas switches in LTD roughly pass through three different phases: initial phase, middle and post phase and erratic phase. The static characteristics acquired by experiments show that the switch in erratic phase with over 300 shots has discharging jitter about 20 to 33 percent. For the switch in middle and post phase with more than 100 shots, by analyzing the constant voltage data of static discharging of the switch of 0.08 MPa, the self-discharging probability is respectively presented with Gauss and Weibull distribution model; Further more, for 42 kV operation volt