The rapid growth of multimedia group-based applications motivates research into QoS guaranteed multicast network. In the multicast network carrying traffics of different QoS requirements, bandwidth fragmentation is considered as a bottleneck for traffic admission fairness and efficient usage of bandwidth resource. When the bandwidth fragmentation happens, the chance of accepting high-bandwidth flows is greatly decreased. To prevent bandwidth fragmentation in multicast network, we propose a new approach of bandwidth fragmentation avoided QoS multicast routing in this paper. In our new approach, we integrate active admission control into traditional QoS multicast routing algorithms to gain bandwidth fragmentation immunity. Moreover, an algorithm named dynamic bandwidth allocation with adaptive constraint is proposed and investigated as an example of active admission control algorithm in this paper. Because this algorithm is non-linear and unsolvable by analytical approach, we employ OPNET simulation to study its performance.