On the basis of characteristics of the VUV light source at National Synchrotron Radiation Laboratory(NSRL), a V-coupling-blade Beam Position Monitor (V-BPM) is developed to detect the stability of the low energy beam radiated from an undulator. The monitor is composed of two pairs of V-coupling-blades made of molybdenum. To increase the photoemission currents and improve the response sensitivity of the probe, both V-coupling-blades are inserted in the beam edges at an angle of 10° in horizontal and offset each other along the beam direction, and the bias of ~300 V is adapted to collect the photon-electronics more effectively. It describes the principles and structure features of the V-BPM and discusses the results of performance tests. A V-BPM is mounted on the undulator beamline at the NSRL to monitor the stability of the light source and a series of data on the tests are obtained. The results show that the V-BPM can test the radiation beam position from the new undulator in the improved NSRL in real time and obtained data meet the requirements of monitoring photon-beam stability.