A new method for generating single longitudinal mode operation of TEA CO₂ laser-using four arm cavity to select single longitudinal mode is proposed. The new cavity’s reflectivity character has been theoretically studied, and its mode selectivity is predicted. In experiment, single longitudinal mode TEA CO₂ laser pulses are readily achieved using this method, with laser pulse energy up to 225mJ and single longitudinal mode repetition more than 90%.