Transparent cellulose papers used as substrates have gained increasing attention in the flexible optoelectronics field. However, significant challenges remain in overcoming the hydrophilicity of transparent cellulose papers and endowing them with conductivity for multifunctional applications. Herein, a new avenue for the construction of highly transparent, ionically conductive, and hydrophobic cellulose paper (HCP) via photopolymerization is reported. The photopolymerization of ionic liquid-containing photopolymerizable precursor and allyl cellulose on the surface of allyl cellulose paper yields robust functional surfaces. The obtained HCP displays high transparency (>90% in a wide wavelength range), better surface hydrophobicity (water contact angle ~127°), and high thermostability (onset thermal decomposition ~220 °C). In addition, the HCP exhibits reliable, sensitive and reproducible abilities to external stimulus. Significantly, the ionic conductive HCP has successfully been used for the construction of integrated and flexible optoelectronic devices. This promising strategy provides new insights into tailoring the surface hydrophobicity and functionality of cellulose paper for flexible electronic devices.