To reveal the low growth rate ofAcidithiobacillus ferrooxidans, a stochastic growth model was proposed to analyze growth curves of these bacteria in a batch culture. An algorithm was applied to simulate the bacteria population during lag and exponential phase. The results show that the model moderately fits the experimental data. Further, the mean growth constant (K) of growth curves is obtained by fitting the logarithm of the simulating population data versus the generation numbers with the different initial population number (N₀) and initial mean activity of population (A₀). WhenN₀is 300 and 700 respectively, the discrepancy ofKvalue is only 0.91%, however,A₀is 0.34 and 0.38 respectively, the discrepancy ofKvalue is 19.53%. It suggests that the effect ofA₀on the lag phase exceedsN₀, though both parameters could shorten the lag phase by increasing their values.