In this paper, we develop a method to measure the refractive index of liquid at different temperatures applying the femtosecond laser frequency comb. The measurement principle is to test the phase changes of the measuring signal caused by light traveling through the liquid, then calculating the refractive index according to the phase difference between water and air. The experiment on measuring the water refractive index at temperature from 20 o C to 60 o C was performed, and the measuring results show a satisfactory accuracy.