To examine the application prospect of system dynamics approach in pest population optimal management and prediction, population dynamics of the 2nd generation of Cnaphalocrocis medinalis Guenee was investigated in this paper. Influences of the dominant predators including spiders and ducks in rice paddy field on Cnaphalocrocis medinalis Guenee were analyzed and a frame model was established. Using the system dynamics method, population dynamics of the Cnaphalocrocis medinali Guenee was simulated and forecasted under different scenarios. Results indicated that with a number of 100 initial eggs and a defined spider density in 12.623 ind/100 hill, the estimated adult Cnaphalocrocis medinalis Guenee population with ducks predation at a constant rate of 60% was 9.129 ind/100 hill. The activities of the ducks reduced the field ovum of 3rd generation by 6.7%. The main conclusion from the model results was that a more effective suppression of Cnaphalocrocis medinalis Guenee population could be implemented with optimum combining the function of the spiders and ducks in pest control. The results are expectative to provide worthy informations and scientific guides with rapid and convenient ways