Ring laser gyro is usually pumped by high voltage discharge. Negative impedance characteristic of discharge tube can cause instability or oscillation of the current adjustment circuit. In order to optimize the discharge circuit loop, the impedance characteristic of discharge tube in ring laser gyro was researched. The voltage drop was measured under different discharge current, and voltage-current characteristic was obtained through nonlinear curve fitting. And then differential impedance, which was related to adjustment stability, was analyzed. The research can provide guidance for selection of ballast resistor and reduce the power consumption on premise of reliability.