Anisotropy in a crystal structure plays a striking role in determining the optical, electrical and thermal properties of the condensed matter. Here, we investigated in-plane vibrational anisotropy in a two-dimensional (2D) van der Waals (vdW)-layered GeAs narrow-gap semiconductor by combining microstructural characterization and polarization Raman spectroscopy. Interestingly, not only the intensities but also the Raman shifts in all modes evolved periodically with different symmetries as the polarization angle changed continuously, which could be well-analyzed using the Raman tensors and further interpreted from the phonon dispersion relations. More importantly, the temperature-dependent Raman intensities of the Raman modes in the range from 83 K to 823 K gave a thermal-related uniform constant, based on which key parameters, including the thermal expansion coefficient, Grüneisen constant and quasi-particle lifetime, could be directly derived, which were in line with the calculated predictions. This investigation provides a comprehensive understanding of structure-dependent optical anisotropy in 2D vdW-layered GeAs and suggests a new idea for exploring the thermal properties of related materials using temperature-dependent Raman spectroscopy.