The model of the molecular weight distribution (MWD) of polypropylene produced in a loop reactor is established. The simulated MWD data of the polymers produced in steady-state polymerizations agree with the actual data collected from certain plant. The simulated weight-average molecular weight data also agree with the plant data in start-up processes. Furthermore, the model can be used to simulate the influence of the operation parameters on the MWD of the polymers produced in the steady-state polymerizations as well as the dynamic polymerizations. The simulated results of the steady state polymerizations show the MWD width of polypropylene decreases with the increase of both the propylene flow and the hydrogen flow, but increases with the increase of the catalyst flow. We also find the weight fraction of the polymers with short chains increases with the increase of both the propylene flow and the hydrogen flow, meanwhile, a small shift of the MWD curve to long chains can also be found as the catalyst flow increases. In the dynamic polymerizations, simulations indicate the MWD width and the weight fraction of the polymers with long chains all decrease in both of the start-up process and the end-up process of the polymerizations.