To understand the salt-forming reaction process of urotropine(HA) and nitric acid (NA) system and clear the salt-forming mechanism, the change in characteristic absorption peak of main fuctional groups with time in the process of forming mononitrate of HA (HAMN) and dinitrate of HA (HADN) was tracked, monitored and analyzed by on-line infrared spectroscopy technology and the interaction between HA and NA was studied by computational chemistry method. Results show that in the salt-forming process of HA, the reaction of forming HAMN by NA with HA is quick, but the reaction of HAMN to HADN is relatively low. When NA reachs 5.97 mol·L^-1, HADN is only formed. In the IR spectra of HAMN, the inverted peaks appear at 1002, 1236, 690, 806 cm^-1, which are the characteristic absorpption peak of C—N bond in HA. Due to the HAMN is formed, the C—N bond force constant of HA is changed, the absorption peaks at 979, 1024, 1219 cm^-1 and 1259 cm^-1 were appeared on the left and right sides of the inverted peak.When there is HADN precipitation, without adding NA, there are still HADN formation and precipitation and the amount of NA in the solution shows a slow upward trend. In the process of HAMN formation, HAMN adsorbs the NA molecules around it. When the HADN is formed, the NA molecules are released into the solution, and the HAMN molecules are supplied to continue to form new HADN molecule. The optimized structure of HA and NA, HA and H₂O obtained by Gaussian software and calculated binding energy values are obtained, revealling that the NA system of HA is more stable. The results are in agreement with the experimental ones.