In order to analyze the factors affecting the ignition delay time in the gas phase reaction process between hydrazine fuel and NO₂， the reaction process was simulated by using density functional theory. The reaction activities， active sites， potential energy surfaces and reaction rate constants in hydrogen extraction reaction process of hydrazine （N₂H₄）， methyl hydrazine （MMH） and unsymmetrical dimethyl hydrazine （UDMH） were calculated. The results showed that the energy difference between the highest occupied orbital and the lowest vacant orbital of UDMH was the smallest among the three hydrazine fuels， which was 0.20522 eV， indicating that UDMH had the highest activity， so it has the fastest reaction rate with NO₂， which in line with the characteristic of the shortest ignition delay time. The active sites of three hydrazine fuels were identified， N（1） or N（4） for N₂H₄， N（1） for MMH and N（1） for UDMH. The active sites of hydrogen extraction reaction of three hydrazine fuels were calculated， it was found that the reaction barrier of UDMH is the smallest， which is 3.589 kJ ·mol^-1， and the reaction rate constant is the largest， which is 9.81×10⁵ L·s^-1·mol^-1， which is consistent with the shortest ignition delay time， it is concluded that in hydrazine fuel， the smaller the hydrogen extraction reaction barrier with NO₂， the larger the reaction rate constant， and the shorter the ignition delay time.