This work measured the absolute photoabsorption cross section of propyne (methylacetylene, CH3CCH) in the 6-30 eV energy range. The valence-shell oscillator strength of 16.97 and the static dipole polarizability of 35.82 atomic units (5.31 Angstrom(3)) were also derived from the photoabsorption spectrum by the sum-rule method. In the photoionization measurement, the first ionization energy 10.369+/-0.007 eV of propyne and appearance energies for ion fragments were determined. In addition, new vibrational nu(9) and nu(10) progressions associated with the X E-2 ion state of propyne and Rydberg transitions converged to the A E-2 ion state were identified. In addition to the main nu(3) progression with the vibrational interval of 1979+/-39 cm(-1), the nu(9) and nu(10) vibrational structures of the X E-2 ion state were resolved with vibrational frequencies of 586+/-74 cm(-1) and 289 +/- 65 cm(-1), respectively. Moreover, the absolute branching ratios, photoionization quantum yield, and partial photoionization cross sections for C3H4+, C3H2+, and C3H+ production in the 10.3-21.4 eV region were determined. Four shape resonance states were tentatively assigned at 11.2, 12.7, 14.8 and 15.8, and 16.7 eV. Furthermore, a CD3CCH isotopic measurement was taken to examine the reaction pathways of I-I-dissociative photoionization of propyne. According to the evolution of the C3D3+/C3D2H+ ratio, plausible decay mechanisms for H-dissociative photoionization of propyne were proposed.