We study the behavior of Ge–Ge, Ge–Sn, and Sn–Sn vibrational modes in GeSn semiconductors with Raman Spectroscopy. Raman spectroscopy is a rapid, nanoscale spatial resolution and non-destructive approach to accurately determine the composition and strain information of GeSn, and it is thus crucial for the material investigation and device application of GeSn alloys. By using several excitation wavelengths at 532, 633 and 785 nm on a set of fully strained and fully relaxed Ge1-xSnx layers with the Sn composition in the range 2.3 % < xSn < 31 %, all modes are identified and their evolution as a function of strain and Sn content is determined. The Raman shifts of all vibrational modes are found to exhibit the same function versus the composition xSn and in-plane strain ε//, Δω = ωGeSn - ωGe = axSn + bε//, where a is the Sn composition factor and b is the strain shift factor. In addition, the Ge–Sn mode intensity increases with Sn content. It is discovered for the first time that the Sn composition determined from the plot of the intensity ratio of the Ge–Sn mode over the Ge–Ge mode as a function of Sn composition at 785 nm excitation agrees well with that the X-ray Diffraction (XRD) Reciprocal Space Mapping (RSM), offering a novel approach for determining Sn content by Raman spectroscopy. •A set of fully strained and fully relaxed GeSn layers with 2.3 %<xSn<31 % grown by MBE.•Ge–Ge LO, Ge–Sn and Sn–Sn vibrational modes behavior under multi-wavelength Raman.•Analytic fitting of Raman peak position is obtained to estimate xSn and strain.•First time to find the intensity ratios of Ge–Sn and Ge–Ge modes are related to xSn.