Through investigation of tracking infected Spodoptera litura larvae, the epizootic dynamics of S. litura nuclear polyhedrosis virus(SlNPV) in field was studied with different dosages of SlNPV. The results showed that in the range of virus dosages (3.1 x 10(5)-3.1 x 10(8) PIBs.ml-1), the initial infection syndrome was observed after 4 days of treatment, and peaked in 5-7 days. The larvae mortality occurred after 5-6 days of treatment, and peaked in 6-8 days. The prevalence peak of host was basically coincident to the incidence peak. The distribution of diseased incidence of hosts and disease death time could be quite simulated by time-dosage-mortality model(TDM). Hosmer-Lemoshow test showed that the predicted values well fitted with observed data, and t-test indicated the parameters of the model reached significant level(P < 0.0001). The distribution of diseased prevalence was simulated by Holliday model, the regression of function was significant by F-test, and the parameters of the model reached or approached significant level(p < 0.005). TDM model could be used to predict the daily incidence, prevalence, disease death rate of present generation population of host pest.