A high-quality polyene can be obtained by extensive dehydrochlorination of poly(vinyl chloride) (PVC) in a liquid/solid two-phase system. The liquid phase is a tetrahydrofuran solution of PVC containing a small amount of poly(ethylene glycol) with molar mass of 400 g as a phase transfer catalyst. The solid phase is potassium hydroxide particles.The structure of the polyene is polyacetylene-like and has a long conjugated C=C sequence and a narrow dispersity of polyene sequences according to its FT-infrared and Raman spectra. The environmental stability of the polyene was also studied by IR, Raman spectra and elemental analysis. Experimental results demonstrated that the polyene was susceptible to air and could be changed into a material containing high concentrations of hydroxyl and carbonyl groups. The polyene sequences were shortened and its dispersity became broader due to the effect of dioxygen.