Abstract: In this paper, the reaction in solution which have a gaseous result(ⅰ) is chosen as an object of study. By AGA- 100 model quadrupole mass spectrometer, the concentration (X_i) of the gaseous result was tracked and measured at varying reaction time(t). From deducing functional relations of X_i vs. t for varying order of the reaction, the way from which the order (n) of this reaction and rate constant(k) can be obtained is found. By measuring (k) at varying temperature, also the apparent activation energy (E_a) can be gained.The thermal hydrolysis reaction of diazonium salt Cl N_2~+Cl~- was used as an example. To study its kixetics by this method, the result is that n=1, k=5. 41×10~(-4)S~(-1) (55℃) and E_a=8.05×10~4J·mol~(-1). What we get is consistent with those results from documents. So using method of quadrupole mass spectra to study kinetics of a reaction is feasible.