According to the periodical property of temporal coherence g(1)(τ) in a multimode gas laser, a new method of observing the spectral line-shape function and its frequency width of longitudinal mode in a multi-mode laser is proposed in this paper. The investigation shows that the spectral line-shape function of longitudinal mode in the multimode gas lasers is Lorentz line- shape function, and relative frequency width of longitudinal mode is about 107 Hz.