The present invention discloses a method for maintaining quality of service QoS, which includes: identifying a CLP and a service type of a newly received ATM cell, and obtaining a corresponding QoS label; when the QoS label corresponding to the newly received ATM cell is different from a QoS label corresponding to a buffered ATM cell, encapsulating the buffered ATM cell into a pseudo wire PW packet in a concatenation manner and sending the pseudo wire PW packet, where the QoS label corresponding to the buffered ATM cell is used as a QoS label of a PSN transport header of the PW packet, or is used as QoS labels of both the PSN transport header and a PW header of the PW packet, and buffering the newly received ATM cell. The present invention further provides a corresponding provider edge PE device and a corresponding ATM PWE3 system.