We obtain a stable tunable single-longitudinal-mode (SLM) TEA CO₂ laser oscillation by using a Fox-Smith cavity with a Fabry-Prot etalon inserted in the sub-cavity. A numerical model of the reflectivity of the interference cavity is studied. The influence of the detuned angle and the sub-cavity-matching on the mode-choosing is discussed,and the verification experiment is carried out. Finally we get 140 mJ of SLM output energy at 10.6 μm with repetition frequency of single longitudinal mode as high as 90%,and there is no damage on the elements in the cavity.