With the increasing needs of clean energy, supercapacitors, especially the activated carbon (AC) based supercapacitors are attracting great attentions for their theoretically infinite lifespan. However, they always age much faster than expected due to some mechanisms. In this work, we probed the ageing mechanism of AC based supercapacitors systematically by in-depth relevancy analyses of the main components in the supercapacitors after they have been aged at different temperatures for a rather long duration (1296 h) which was specially designed to simulate practical service limit of supercapacitors. On the one hand, XRD, TG-MS, XPS and BET analyses indicated that more than 23% of capacity decrease after high temperature ageing could be ascribed to the degradation of the crystal structure for the AC carbon, the agglomeration of the conductive carbon black, the collapse of the pore structure in the mixed AC layer and the separator, and the peeling off of the carbonaceous layer from the current collector, which was eroded as well during the ageing process. On the other hand, we found that the size of bare and solvated BF4 ions were about 2.86 angstrom and 11.11 angstrom, respectively, with the help of density functional theory (DFT) calculations. So the performance degradation of the supercapacitor could be ascribed to the lowered ion diffusing efficiency in the collapsed pores together with the increased electrical resistance of the whole system and the decreased specific area, which were resulted from all kinds of side-reactions ultimately. In addition, the relatively more serious ageing of the anode might be due to the more fluorochemicals and nitrogen compounds produced by side-reactions in the anodes. (C) 2017 Elsevier Ltd. All rights reserved.