A novel fluorescent phosphor consisting of cellulose and carbon dots (CDs) has been successfully prepared for temperature sensing by the inverse microemulsion method for the first time. Cellulose is dissolved in N-methylmorpholine-N-oxide at high temperature, which can destroy the hydrogen bonds among the cellulose molecules and activate hydroxyls. At the same time, some of the hydroxyl groups are gradually oxidized to carboxyl in this process, which effectively reacts with the CDs, whose surface is functionalized by lots of amide groups. The result from Fourier-transform infrared spectroscopy confirms that there is strong interaction between cellulose and the CDs due to the formation of carbon–nitrogen bonds through dehydration–condensation reactions. In addition, the composite phosphor retains the inherent properties of individual CDs and emits 450 nm blue light, however an interesting excitation dependent phenomenon is observed. Finally, the temperature dependent fluorescent properties of the cellulose/CDs phosphors have been investigated from 200 to 420 K. After liner fitting, the correlation coefficient is calculated to be about 0.99, thus this novel phosphor is expected to be applied in temperature sensing.