To examine the application prospect of system dynamics approach in pest population optimal management and prediction, population dynamics of the second generation of Cnaphalocrocis medinalis (Guenee) was investigated in this paper. The influence of dominant predators of C. medinalis (Guenee), including spiders and ducks in rice paddy field, were analyzed and a frame model was established. Using the system dynamics method, population dynamics of the C. medinalis (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 C. medinalis (Guenee) population with ducks predation at a constant rate of 60% was 9.129 ind/100 hill. Duck activities reduced the field ovum of 3rd generation by 6.7%. The main conclusion from the model results was that a more effective suppression of C. 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 useful information and scientific guides.