A liquid scintillator (LS) is developed for the Taishan Antineutrino Observatory (TAO), a ton-level neutrino detector to measure the reactor antineutrino spectrum with sub-percent energy resolution by adopting Silicon Photomultipliers (SiPMs) as photosensors. To reduce the dark noise of SiPMs to an acceptable level, the LS has to work at -50 degrees C or lower. A customized apparatus based on a charge-coupled device (CCD) is developed to study the liquid samples' transparency in a cryostat. We find that the water content in LS results in transparency degradation at low temperatures, which can be cured by bubbling dry nitrogen to remove water. Adding 0.43 g/L ethanol as co-solvent cures the solubility decrease problem of the fluors PPO and bis-MSB at low temperature. Finally, a Gadolinium-doped liquid scintillator (GdLS), with 0.1% Gd by weight, 2 g/L PPO, 1 mg/L bis-MSB, and 0.43 g/L ethanol in the solvent LAB, shows good transparency at -50 degrees C and also good light yield.