Optical third-harmonic generation in Cesium medal vapor is discussed in this paper.The theoretical analysis shows that there is a two-photon resonance enhancement effect for nonlinear susceptibility Ｘ(3)(3ω)of Cesium at wavelength 1.0790 μm.The temperature which is necessary to achieve phase matching is given. It's the first time that the third-harmonic spectral line(359.7 μｍ) of the fundamental wave 1.0790 μm in Cesium vapor is photographed experimentally.