A novel temperature sensor of photonic crystal fibers (PCF) based on liquid ethanol filling with refractive index sensitive to temperature is proposed. The temperature properties of the sensor were investigated by the full-vector finite element method with a perfectly matched layer. Theoretical calculations show that filling the air-holes of a PCF with ethanol will enhance the temperature influence on the mode field. Both the effective refractive index and the confinement loss decrease with increase of temperature. After filling the liquid ethanol, the mode field infiltrates more through the cladding of fiber as the wavelength increases, and becomes more sensitive to temperature. Under the condition of the same hole spacing, the larger the duty ratio and the longer the input wavelength are, the stronger are the dependence of the effective refractive index and the confinement loss on temperature. The effective refractive index basically reduces linearly when temperature increases. When the wavelength λ=1550 nm, hole to hole spacing Λ=2.3 μm and duty ratio d/Λ=0.7 (d represents the hole radius), the confinement loss reduces from 3.5×102 dB/m to 22 dB/m as the temperature changes from -20 ℃ to 70 ℃.