The bus-bar voltage failure is the most important starting condition of the automatic back-up power supply device.The bus-bar PT-breaking may effect the voltage acquisition, so the most automatic back-up power supply device equipment the PT-breaking detection function. In this paper, the judgment and treatment methods of the existing automatic back-up power supply device is analyzed firstly. Then, it discusses the existing problems and gives the corresponding technology improvement measures. The test and running experience show that the improvement measures is effective and reliable.