G-rich aptamers have been widely applied to develop various sensors for detecting proteins, small molecules, and cations, which is based on the target-induced conformational transfer from single strand to G-quadruplex. However, the transforming process is unclear. Here, with PW17 as an aptamer example, the forming process of G-quadruplex induced by K+ is investigated by circular dichroism spectroscopy, electrospray ionization mass spectroscopy, and native gel electrophoresis. The results demonstrate that PW17 undergoes a conformational transforming process from loose and unstable to compact and stable G-quadruplex, which is strictly K+ concentration-dependent. The process contains three stages: (1) K+ (<0.5 mM) could induce PW17 forming a loose and unstable G-quadruplex; (2) the compact and stable K+-stabilized G-quadruplex is almost formed when K+ is equal to or larger than 7 mM; and (3) when K+ ranges from 0.5 mM to 7 mM, the transformation of K+-stabilized PW17 from loose and unstable to compact and stable occurs. Interestingly, dimeric G-quadruplex through 5'-5' stacking is involved in the forming process until completely formed at 40 mM K+. Moreover, the total process is thermodynamically controlled. With PW17 as a sensing probe and PPIX as a fluorescent probe for detection of K+, three linear fluorescent ranges are observed, which corresponds to the three forming stages of G-quadruplex. Clarifying the forming process provides a representative example to deeply understand and further design aptamer-based biosensers and logic devices.