The invention discloses a method and a system for service quality control in a service delivery network. A CDN management system provides different numbers of copies and different placement positions for contents of different service grades according to service grade information of various contents maintained by a service management system when pre-distributing content copies. According to the invention, the service management system controls copy pre-distribution of the CDN management system to enable contents of different service grades to obtain copy placement qualities of different grades, thereby saving storage resources of Cache server nodes on the whole and ensuring that better services can be provided for higher-grade contents. In addition, cache and service of the Cache server nodes can be controlled by the service management system so that contents of different service grades can obtain cache qualities and service qualities of different grades, thereby ensuring high-quality service for a specific content.