Thiol-ene photopolymerization has been a versatile tool for polymer synthesis. However, deep photopolymerization of the thiol-ene system still remains a great challenge due to the light intensity gradient. Using near-infrared light as an excitation source and upconversion nanoparticles as internal lamps, we report a generic and efficient strategy to realize deep thiol-ene photopolymerization. More than 8 cm of photopolymerization depth combined with high functionality conversion was obtained in 40 s. The maximum temperature of about 110 degrees C during upconversion nanoparticle (UCNP)-assisted photopolymerization is comparable to or lower than that of some reported frontal photopolymerizations applied in the preparation of functional polymer composites and biomaterials.