Spatial index trees constructed in wireless sensor networks are used to determine the sensors which can participate the query accurately and quickly. Most of index trees are constructed based on the parent--child node relation in network structure like routing tree, in which message sending for parent node selection will consume more energy. Due to energy being the important factor considered in wireless sensor networks, we design an energy-efficient index tree based on grid division and minimum energy merging principle in the skewness distribution of sensor nodes. Multi-region aggregation queries are carried on in our proposed index tree, which mainly focuses on region re-combination. Experimental results show that the energy consumption for multi-region aggregation queries are reduced compared to the original index tree