Abstract: To study the tectonomagnetic effect in the reservoir vicinity,2 21 sites,about 2-5 km apart,have been set up around the Miyun reservoir. The distances between geomagnetic sites and the reservoir bank are some tens of meters to 15 km. Using G-816 and G-826 magnetometers,geomagnetic total intensity was observed once each season during 1983-1987. Analysis of geomagnetic data and the data of water storage of the Miyun reservoir shows that the correlation coefficients between the geomagnetic changes and the variations in water level and water volume are - (0.28 + 0.22) nT/m and - (0.35 + 0.31)10-8nT/m3 respectively. It shows that there is a stronger negative correlation between the geomagnetic changes and the variations in the water storage. This rather strong negative correlation is probably a reflection of the piezomagnetic effect of the magnetic rock under the vicinity of the Miyun reservoir. Because according to the experimental results,the tectonomagnetic experiment around the reservoir is a rather good simulation of the seismomagnetic phenomena,it is believed that geomagnetic observations with a dense network in a seismically active area may catch the information of seismomagnetic precusors.