In the low permeability reservoir where nature fractures developed，anisotropy has a great impact on areal sweep efficiency of water flooding. However the current methods have not taken the influence of anisotropy into consideration. The frontal water isosaturation interface was introduced，and the concept of areal sweep efficiency was defined. A new method for evaluating the areal sweep efficiency was established in the low permeability reservoir using streamline simulation technique. The results show that the calculated areal sweep efficiency while water breakthrough by this method is the same as that by reservoir engineering method. Based on the data from a block in Changqing oilfield，the method was applied to make the charts of the areal sweep efficiency under various aeolotropies in quadrate inverted nine-spot pattern，and the areal sweep properties under different aeolotropies were analyzed. Improvement strategies of easing areal sweep contradictions were proposed and the range of the increased areal sweep efficiency after the adjustment was predicted and the corresponding charts were made.