This work applies THz-TDS and ab initio simulations to explore the two highly similar polymorphs of aspirin. The results show that, even for these two nearly identical crystal structures, THz-TDS can reveal spectral differences based on its sensitivity to inter-and intramolecular forces. With the help of DFT simulations, slight differences observed for the aspirin polymorphs and the spectra provide further solid evidence of the intergrown nature of the two forms. A number of calculated physical properties are compared for the two forms and most of them are found to be quite similar, reflecting the high degree of structural similarity. However, form II shows a relatively larger vibrational entropy, which yields a lower free energy than form I at temperatures above 75 K. Larger parameters for the baseline fitting of the THz-TDS spectra also indicate a higher degree of structural disorder for form II, consistent with established expectations for this polymorph. Further computational work on a supercell structure incorporating defects would be very valuable to investigate the influence of structural imperfections in the system and could also add insight into possible phase transitions between the aspirin polymorphs. However, a highly efficient computational plan will be required as well as a large amount of computational resource to pursue this work further.