A system for ensuring quality of service (QoS) in a virtual private network and a method thereof are provided. The system includes a logical bearer network, which is formed by connecting label switch paths configured with preserving bandwidth to routers, and is dedicated to transmit QoS service data; and a bearer control network, which is used to maintain logical bearer network, allocate route, mark service priority, and route service data to opposite end. The method includes the steps of: A. constructing a logical bearer network to transmit QoS service data by configuring label switch paths with preserved bandwidth; B. providing a centralized resource controller to manage resources of the logical bearer network; C. if QoS service data is to be transmitted, marking service priority in QoS field of the routing labels of multi-protocol label switch data packets, and routing service data to the opposite end.