In the investigation of gas laser pumped by electric discharge, the stability of laser output energy in repetition rate mode is the foundation of increasing its output average power and its application in practice. By adjusting the velocity of gas flow and optimizing the operating condition of laser, the output energy stability of laser in repetition rate mode is studied experimentally with joule level discharged-pumped pulsed HF laser, and the optimal operating condition of laser is obtained. The experimental results show that laser output energy decreases quickly with the repetition rate increasing; and increasing gas circulating velocity is beneficial to improve the discharge stability and the stability of laser output energy in repetition rate mode. When the gas mixture circulates at 3.5 m/s flowing velocity, laser can operate at repetition rate from 1 Hz to 50 Hz and operating stability keeps well. Under these conditions, each laser pulse of 130 mJ is obtained stably, and the fluctuation of output energy is about 5%.