Owing to the multiple redox couples of Nb5+/Nb4+ and Nb4+/Nb3+, Nb-based compounds have attracted great attention to be promising high-capacity anode materials for rechargeable batteries. Here, K6Nb10.8O30 groove nanobelts (GNB) are synthesized through heat-treating the adjustable electrospun potassium niobate nanofibers, thereupon the structural change in the lithiation and delithiation is evidently imaged via in situ transmission electron microscopy (TEM). From in situ observations, the K6Nb10.8O30 GNB, in virtue of its stability, is ascertained to be adopted as anode material in lithium-ion batteries (LIBs). Evaluated as lithium storage host, GNB outstrip nanowires (NW) in cyclicity and in reversible capacity. Even after 1000 cycles, the retention capacity of K6Nb10.8O30 GNB is as high as 69%. Furthermore, the lithium-storage mechanism is also investigated via in situ X-ray diffraction (XRD). It is proved that the phase transition takes place from Li-poor K6Nb10.8O30 phase to Li-rich Li22K6Nb10.8O30 phase via two steps. In addition, the results obtained from ex situ TEM and ex situ X-ray photoelectron spectroscopy (XPS) also prove that the behavior of lithiation and de-lithiation for K6Nb10.8O30 GNB is highly reversible, suggesting that it can be a possible anode material.