RNA/DNA sequences rich in guanine (G) can form a 4-strand structure, G-quadruplex, which has been extensively researched and observed in mammalian, fungi, and plants, with in vivo existence in eukaryotic cells. Compared with DNA quadruplex, the potential existence of RNA quadruplex appears to be generally rare; however, it is believed by some researchers to be more inevitable in vivo and speculated to play an important role where it exists. Recently, researches concerning the function of G-quadruplexes in RNAs commence, making much progress. However, there is no available review particularly focusing on RNA quadruplex till now as we know. Therefore, we decide to give a review to comprehensively summarize research progress on it. This review highlights the diverse topologies for RNA quadruplex structure and its effect factors; outlines the current knowledge of RNA quadruplex's physiological functions in biological systems, especially in gene expression; and presents the prospects of RNA quadruplex.