For voltage issue caused by large-scale wind farm integration into grid， this paper introduces the power relationship of doubly-fed induction generator（DFIG） and summarizes the research results on the inherent reactive power capability of DFIG. The power-angle characteristic of DFIG and the impacts of static stability constraint and grid code on the reactive power regulation of DFIG are analyzed. Based on these， reactive power regulation capability of DFIG with constraints， such as operation limit， static stability， and grid code， is analyzed. By simulation of actual DFIG， it is shown that the reactive power regulation capability of DFIG considering multi-constraints is closer to the actual. Also， it can help DFIG take part in the grid voltage regulation in wind power integrated region according to accurate reactive power regulation capability of DFIG.