Previous experiments in different media such as gas phase (n = 1-13) , solution (n = 2-10, 12) , and neon matrix (n = 6-12) have observed the absorption spectra of several H-(C≡C) n -H chains and demonstrated their size-dependent electronic and optical characteristics. Also, there were some theoretical studies on the size dependence of the electronic excitation energies and oscillator strengths of the H-capped carbon chains . But, perhaps limited by the analytical means at that time, almost all the reports were merely on location and assignment of the absorption band, and there is still a lack of comprehensive analysis of the size-dependent optical characteristics. Furthermore, it is of great theoretical and practical significance to predict hypothetical carbon allotropes by extrapolating the size dependence of the properties of finite carbon chains.