In the condition of connecting large scale doubly-fed induction generators (DFIGs) into weak grid, the closely coupled interactions between wind generators and power grid becomes more severe. Some new fault characteristics including voltage phase angle jump will emerge, which will influence the power quality of power system. However, there are very few studies focusing on the mechanism of voltage phase angle jump under grid fault in a weak grid with wind turbine integration. This paper focuses on the scientific issues and carries out mechanism studies from different aspects, including mathematical deduction, field data analysis and time domain simulation. Based on the analysis of transient characteristics of DFIGs during the grid fault, this paper points out that the change of terminal voltage phase angle in DFIGs is an electromagnetism transition process, which is different from conventional synchronous generator. Moreover, the impact on transient characteristics of voltage phase angle are revealed in terms of fault ride through (FRT) control strategies, control parameters of current inner-loop of rotor-side converter and grid strength.