Based on the framework of density functional theory，we employed the first-principles molecular dynamics to simulate the thermal decomposition of liquid nitromethane and obtained the time-evolution for the population of decomposition productions. We discussed the three possible reactions at the initial stage of thermal decomposition: intramolecule proton transfer reacton, intermolecule proton transfer reaction, and C—N bond rupture. We also studied the dynamic behavior of thermal decomposition of liquid nitromethane under different density （pressure） conditions. We found that the thermal decomposition of liquid nitromethane sensitively depends on the liquid density of pressure. A possible explanation was given.