When service system is under DDoS attacks, it is important to detect anomaly signature at starting time of attack for timely applying prevention solutions. However, early DDoS detection is difficult task because the velocity of DDoS attacks is very high. This paper proposes a DDoS attack detection method by modeling service system as M/G/R PS queue and calculating monitoring parameters based on the model in odder to early detect symptom of DDoS attacks. The proposed method is validated by experimental system and it gives good results.