The G-quadruplex is one of the most important DNA secondary structures and has elicited widespread interest in this field of scientific research. As G-quadruplex studies have progressed, its significant roles in a variety of biological process have been revealed, including telomerase activity, genome stability, DNA replication, and oncogene expression. In addition to its biological functions, as a special DNA quadruplex structure, it also has potential for other important applications like asymmetric catalysis, probing for important biomarkers, acting as a vehicle for drug delivery, or as a structural regulator for some nanostructure designs. Here, we discuss its application to aspects of biological target detection and drug delivery. Its utility in biological target detection mainly results from its special DNAzyme activity. We would summarize some typical systems, utilizing G-quadruplex as signal readout unit, for various target detection, such as SNP, miRNAs, ions, transcription factors and so on. On the other hand, its use in drug delivery always relies on probes' structural change, as the switch of drug release. Here, we would also exhibit some typical drug delivery designs.