Compared with many preventable epidemics, how did a relatively insignificant disease like SARS develop into an international scare? This article describes the application of system dynamics to understand the SARS epidemic in Beijing. The powersim model simulates the structure of transmission dynamics and factors that impact the epidemic. Here, the probable impacts of changes in the system delays, including delays to quarantine, delays of disease diagnose, and the authorities’ epidemic information transmitting delays, are discussed. The model aims to present detailed understanding of delayed feedback mechanisms inherent to eliminate the misperceptions of basic dynamics, and then to design high leverage policies for preventing SARS. The article concludes that an open and transparent public information system is the most powerful weapon to curb SARS panics. The government’s prompt epidemic information feedback system and relatively instant strong quarantine policies have substantial impacts on containing SARS epidemic.