The invention discloses a guarantee method and a system for stream media file quality of service (QoS). The method includes that a preset media proxy distributes local address information for a cache which stores resources needed by user equipment (UE), and tells a packet data network gateway (P-GW) the local address information; the P-GW sets a dedicated bearer and carries out bearer binding by utilizing the local address information; and the media proxy transmits media stream between the UE and the cache through the bearer according to correspondence of the address information of the cache and the local address information. By means of the technical scheme of the guarantee method and the system for the stream media file QoS, the problem that the amount of internet protocol (IP) filter standard exceeds the amount limited by an existing protocol, namely 15, when thin film transistor generating is carried out on caches placed at different positions in an existing IP multimedia subsystem (IMS) peer-to-peer (P2P) content distribution service (CDS), and the effect of reducing the amount of IP filter standards in the TFT can be achieved.