Abstract: The utilization of the wind and solar complementarity presents an effective approach to mitigating the uncertainty of renewable power output. The present study proposes complementarity indexes based on the fluctuation and intermittency of the renewable power output, in order to address the limitations of current indexes. The proposed indexes are subsequently employed to assess the complementarity of wind and solar in Ulanqab. Analysis results demonstrate that the northern region in Ulanqab exhibits higher degree of complementarity compared to the southern regions. The mitigation of fluctuation and intermittency of renewable power output cannot be maximized when a single renewable resource accounts for a higher proportion of the installed capacity in a hybrid system. The proposed indexes can be utilized to optimize the wind and solar installed capacity ratio to achieve the optimal complementary effect.