The SnO2 material has been considered as a promising lithium-ion battery anode candidate, and recently, the importance has been increased due to its high performance in sodium-ion batteries. Remarkably, the SnO2 lithium-ion battery anode usually shows extra specific capacity that greatly exceeds the theoretical value. Partial reversibility of conversion reaction has been commonly considered to contribute the extra capacity, however, this has not clearly solved due to the indirect experimental evidences. Here, a realistic ex situ transmission electron microscopy (TEM) analysis technique was developed to reveal the origin of the extra capacity. We demonstrate that reactions of Li2O phase contribute to the extra capacity and the reverse conversion reaction of SnO2 hardly occurs in the real battery system. This work provides significant implications for establishing an accurate electrochemical reaction mechanism of SnO2 lithium-ion battery anode, which may lead to inspiration on enhancing performance of the SnO2 anode in lithium- and sodium-ion batteries as well. Furthermore, the robust ex situ TEM experimental approach we have introduced is extensively applicable to analyses of various battery electrode materials.