As RFID technology being applied more and more widely, its security and privacy problems are increasingly called into question. Means of RFID attack emerge in endlessly and RFID security becomes a relatively new research area. This paper makes research on one of the attack methods–-Denial of Service (DoS). We thoroughly expound and analyze the active jamming to the RFID system which, mentioned as Active Jamming Attack later, is a form of DoS. Through theoretical derivation and simulation, the most effective active jamming approach is summarized, which is also demonstrated by presenting experimental results. This work may provide a reference for RFID designers to prevent or weaken this type of DoS attacks to the greatest extent.