Multipath routing is an important and promising technique to increase the Internet's reliability and to give users greater control over the service they receive. Currently the interdomain routing protocol limits each router to using a single route for a destination network, which does not satisfy the diverse requirements of end users. In this paper, in order to support the effective and efficient multipath routing, we propose a multipath interdomain routing system(AMIR), which not only provides more novel paths but also realizes a new AS-level routing scheme. In the control plane, the topology information is collected from neighboring ASes around the primary path, and based on this topology, multipath of the special node pairs is calculated by our multipath discovery algorithm. In the data plane, we use the interdomain source routing to forward the packets. Experiments with Internet topology and routing data demonstrate that AMIR is practical and feasible, and offers tremendous flexibility and diversity for path selection with reasonable overhead.