Abstract: Active network is a novel network conception, which can solve the inadaptability and inflexibility of network infrastructure, innovating network services easy and speedily. Active network is a programmable network which users can inject customized programs into active nodes and run it to perform customized computation on the user’s, messages passing through them. The paper here introduces a new operating system, an Active Operating System for Router （AOSR）, which can satisfy the Quality of Service （QoS） required by applications as much as possible and improve resource utilization of network node. It provides an environment for active nodes for loading and running the user’s customized programs, while forwarding the data packets safely and rapidly.