A monitoring system based on wireless sensor network is designed in order to solve the problem of elder monitoring. This project has designed a wearable ECG monitoring module to collect the real-time data of ECG signal, and uses the Embedded Zero tree Wavelet ECG compression algorithm to compress the ECG data. It builds a distributed wireless sensor network, and uses the three points location algorithm to achieve the positioning tasks of the elders. Then the system sends the physiological data and the location message to the software on monitoring center to analyze, display and record. The routing line is selected through analyzing the calculated results. The experimental results show that this system performances excellently in ECG tests and positioning tasks of the elders. Therefore, it contributes to the monitoring of elder, and meets the demand of aging.