With the large number of distributed power sources connected to the grid, the voltage over-limit and stability problems of the distribution network are more serious, and the emergence of distribution network phasor measurement unit(D-PMU) provides a new means for real-time online monitoring of the stable operation of the distribution network. This paper proposes an online evaluation index for voltage quality that is suitable for D-PMU and comprehensively reflects the voltage stability and over-limit of the distribution network. Firstly, on the basis of analyzing the adaptability of the on-line voltage stability index in distribution network, it shows that the existing indexs can reflect the voltage stability of the distribution network to a certain extent, but it is difficult to reflect the common voltage out of bounds problem. Secondly, through constructing a continuous step simulation function to simulate the voltage out of bounds, an online evaluation index is proposed. The index can be quickly calculated by the measurement information of the D-PMU, which can well reflect the common voltage over-limit problem in the distribution network. Finally, the simulation is verified by a simple four-node power distribution system and an actual 20 kV distribution network model.The results show that the proposed index can reflect the voltage limit and stability of the distribution network timely, and verify the correctness and effectiveness of the index.