Huang and Rhys published a quantum theoretical treatment to the light absorption in F-centre in solids, which has been widely recognized as the first detailed quantum-mechanical calculation. In the Huang-Rhys’s seminal theoretical treatment, they derived a dimensionless factor to characterize electron-phonon coupling strength which was named later as Huang-Rhys factor. Since then, Huang-Rhys factor has been generally accepted. In this short review, the physical nature of Huang-Rhys factor and several application examples in solids are introduced and presented in memory of the hundredth anniversary of Prof. Kun Huang. Due to limited publication space and my personal understanding on Huang-Rhys factor, only several cases including GaN, 2D WS₂ monolayer semiconductor, inorganic CsPbBr₃ perovskite nanosheets and NV centers in diamond, in which the extremely-weak and medium strong coupling between electron (exciton) and phonon occur, are discussed in this short review.