A dual-mode wireless sensor network system including a local wireless sensor, a local event processing device, and a remote event processing device is provided. The local wireless sensor detects and announces an abnormal event. The local event processing device and the remote event processing device receive the abnormal event announcement. When the local wireless sensor detects the abnormal event, the local wireless sensor encrypts an abnormal event message related to the abnormal event with a regular mode key and transmits the encrypted abnormal event message to the local event processing device. When the local wireless sensor does not receive a response message from the local event processing device, the local wireless sensor encrypts the abnormal event message with a special mode key and transmits the encrypted abnormal event message to the remote event processing device. Thereby, the abnormal event can be successfully announced even with highly protected privacy.