Due to resource limitation, it is an optimum solution for intrusion detection in mobile ad-hoc networks (MANETs) that all nodes investigate system and user activities while a subset of network nodes are selected as monitoring nodes to monitor and detect network packets. In this paper, a selection scheme of monitoring nodes called event triggering based adaptive selection (ETBAS) is proposed, which introduces the mechanism of monitoring states and the method of event triggering. In ETBAS high connectivity and energy resource are balanced for adaptive selection scheme, which is adaptable to dynamically changing networks and enhances the network lifetime. Then a new intrusion detection architecture for energy-constrained mobile ad-hoc networks based on ETBAS is designed, in which the network detection module is optional, monitoring nodes are selected by activating the network detection model to work. Experiments show that ETBAS is effective and the model works perfectly.