Current BPEL engine products are difficult to meet the highly concurrent demands of increasing mission-critical business processes application. We follow the ideas of SEDA and propose a new architecture for high-concurrency BPEL engine, which we call SEDA4BPEL. In SEDA4BPEL, the implementation of BPEL related web services protocols is encapsulated into four primary event-driven stages, to provide independence, isolation and modularity. We also introduce two controllers to manage excessive concurrent process instances. We present the SEDA4BPEL design and the implementation of a BEPL engine based on this architecture. The evaluation results show that SEDA4BPEL applications exhibit high performance and robustness when handling massive concurrency.