We consider the effect of the partially coherent finite duration of excitation laser pulse and instrumental finite response time oil the data fit of quantum beats in isolated molecules．By using a Gauss response mode1 we can simulate the efect of both partially coherent laser pulse and instrumnental response time．We show that Gauss response has the biggest efect oil the first severel peaks of the quantum beat decay．but less Oil the subsequent peaks if quantum beat period js mnch greater than Gauss response width．This approach is applied to analyze quantum beats of S02 in the range of 224~235 nm We find that these quantum beats come from the couple of SO2C(1B2) state with the high vibrational state of the ground state．