Laser-induced breakdown spectroscopy is a new technology of elementary analysis, and it will be used in coal analysis. The delay time of signal is an important parameter of spectral analysis. A LIBS system was set up and three kinds of coal (Jiangxipingxiang coal, Shanxixishan coal and Guizhoupingzhai coal) were chosen for this investigation. The spectra in the range of 240 to 250 nm and 275 to 290 nm of the each three coal samples are shown and they record several spectral lines of components such as C, Mg and Si according to the NIST database. The temporal evolution of SNR of spectral line was obtained, and the value of SNR increased with the time delay, then decayed. The temporal evolution of SNR was different as the coal, element and spectral line differs. Finally, optimum delay time of each spectral lines of elements in the coal samples was calculated according to the biggest value of signal-to-noise ratio, and the relation between the characteristic of coal, element, spectral line and the op timum delay time was analyzed.