As one of the most serious power quality problems, voltage sag has been paid extensive attention by experts and scholars at home and abroad. In the characteristics of voltage sag, in addition to sag magnitude and duration, the influence of phase shift cannot be ignored as well. In this paper, the necessity of the research on phase shift is analyzed. Then the calculation method of phase shift is proposed, and through the data processing and analysis of sag events acquired from the online power quality monitoring systems in several areas of China, the distribution characteristics of phase shifts are given. Finally, the switch mode power supplies (SMPS) and ac coil contactors are chosen as the sensitive equipment, and the experiments on them are given to discuss the influence of phase shift. The voltage-tolerance curves based on different phase shifts are proposed. According to the above analysis, it is quite necessary to consider phase shift as an important voltage sag characteristic and research on them in the voltage sag evaluation.