In order to deeply understand the detection mechanism of determining the components content of liquid steel using laser-induced breakdown spectroscopy, the 45# steel is used to study the different properties between liquid state and solid state which is cooled after solidification. It indicats that the direct measurement stability of liquid steel is poorer than that of the solid steel. The spectrum intensities of liquid steel are significantly stronger than those of the solid steel and the plasma characteristics of them are significantly different. Five iron atomic spectra lines are used to calculate the plasma temperature and a silicon atomic spectra line of 390.55 nm is chosen to estimate electron density. The results show that the plasma temperature and electron density of liquid steel are both stronger than those of the solid steel. The differences of temperature and physical form contribute the differences of plasma and spectral characteristics between liquid steel and solid steel.