In order to solve the discharge instability problem caused by bulk mass and high gas pressure in a fast axial flow (FAF) CO2 laser, a preionization device is presented. The device is installed outside the laser discharge tube and the preionization electrode includes an anode of laser discharge tube and a 35 mm far copper collar. A multivibrator, a RC charge-discharge circuit, and an SCR are combined together to make up the preionization circuit with the discharge frequency of 10～20 kHz for the purpose of strengthening the initial electron density and reducing the laser firing voltage. The experimental result indicates that the preionization device decreases the laser working voltage by 1.69 kV and voltage varicance by 2.6 kV. Such a preionization device can meet the preionization requirement of an FAF CO2 laser and is an effective tool for improving laser′s discharge stability.