The Laser-induced breakdown spectroscopy and spatial confinement method were used to study the optimal detection delay time of the characteristic spectral lines of different elements, to establish a plasma thermal expansion model, to simulate the thermal expansion transient process of the plasma at different spatial confinement depths and times, and to analyze the effects of changes in ambient air pressure, temperature and humidity on the results. Laser-induced breakdown spectroscopy test experiments were carried out on the samples of the traditional Chinese medicine yucca with different spatial confinement depths, using Mg Ⅰ 285.12 nm and Ca Ⅱ 396.84 nm as the characterized spectral lines to analyze the effects of changes in spatial confinement depth and time on the intensity of the spectral line, as well as the effects of changes in excitation light output characteristics on the spectral intensities were also explored. The spectral line intensities were significantly enhanced at a laser energy of 30 mJ, a repetition frequency of 1 Hz, and an optimal spatial confinement depth of 1.9 mm, and the optimal detection times were 0.5 μs for Mg and 1.2 μs for Ca, which indicated that the optimal detection times for different elements with the same spatial confinement depths were different for the enhancement of the spectral lines.