Abstract: The present disclosure provides an event stream processing system, comprises a gateway device and an external module. The gateway device comprises an event processing engine, and the external module comprises external processor. The event processing engine comprises an event grouping unit, a catch-collector, a processor and an event generator. The event processing engine processes a plurality events of the event stream corresponded to a rule. The event grouping unit groups the events corresponded to the rule. The catch-collector couples to the event group unit, configured for storing a first group event. The processor couples to the event group unit, configured for processing a second group event. The external module calculates the first group event and generates a first processing result. The event generator integrates the first processing result of the first group event and a second processing result of the second group event and generates a derived event.