Disclosed is a method for triggering retransmission in a multicast system, including a base station eNB setting a retransmission condition based on a channel state indication and transmitting the set retransmission condition to a user equipment UE; and the UE comparing the current channel state indication of the UE with the received retransmission condition when receiving an error block, and feeding back a NACK signaling to the eNB trigger a retransmission when the retransmission condition is satisfied. The method according to the present invention is to restrict the trigger of the NACK signaling, by setting the condition of the UE feeding back the NACK signaling when the UE received the error block. Then the feedback signaling on the uplink channel would be restricted, and the retransmission probability would be limited in an appropriate region as in Rel 6 HSDPA, therefore the optimal ratio of efficiency over power would be achieved. Consequently, the number of NACK signaling is restricted so as to avoid the uplink overload. Meanwhile, the retransmission probability is limited so that the optimal ratio of efficiency over power may be achieved.