Abstract: ADN (ammonium dinitramide)-based liquid propellant is an emerging environmental friendly mixture with reduced toxicity, which can be considered as a substitute for hydrazine. In order to diagnose the combustion process of ADN-based liquid propellant, it's necessary to develop nonintrusive means to analyze elemental content in real time. Measuring system of absolute radiation spectrum with high-resolution was established, which could be used to obtain laser-induced breakdown spectra (LIBS) of ADN-methanol-water propellant after optimizing experimental parameters. Thus, time evolution characteristics of elements H, N, O in the propellant could be analyzed. Time evolution curves of electron temperature and density of laser-induced plasma were obtained by the methods of Boltzmann slope and Stark effects, respectively. The initial electron temperature and electron density of propellant are 68 000 K, 1.6×10¹⁹ cm^-3, respectively. Besides, the curve indicates relaxation time through exponential decay function fitting:270 ns of electron temperature and 90 ns of electron density. Finally, the ignition of ADN-based propellant had not be observed under this experimental condition.