Abstract: Global Navigation Satellite System-Reflectometry (GNSS-R) technology is a new type of water level measurement that has the advantages of all-weather, all-time and high temporal resolution compared to traditional water level measurement methods. However, GNSS-R based on geodetic receivers has high installation and operation and maintenance costs, which restricts its large-scale application. To address this problem, this paper develops a multi-antenna array method for BeiDou-R water level measurement. The method consists of four low-cost antennas arranged at equal intervals to form a multi-antenna array, which is placed near water for monitoring, and then the collected BeiDou satellite data are processed to obtain the water level changes. The experimental results show that the low-cost device can effectively use data from a wider range of elevation angles, and the accuracy of a single antenna is in the range of 1.2～2.2 cm. By fusing the inversion results of the four antennas, the water level accuracy reaches 1.1 cm, an improvement of about 50%. This research result not only verifies the feasibility of the multi-antenna array water level measurement method, but also highlights the superior performance of BeiDou-R technology in the field of water level monitoring.