Abstract: In order to deduce that the bending loss was a function of curvature radius and temperature,WKB method was used to solve the number of modes of non-uniform straight fibers, the effective refractive index, the cut-off of propagation constant of bending fibers and thermo-optic effect of fibers were introduced to the number of modes of non-uniform straight fiber also.Choosing multimode graded-index optical fibers to get a temperature sensor, the sensitivity of the sensor was 0.1/℃ and the temperature sensing range was 10℃～70℃.The results indicate that the smaller the curvature radius and the temperature, the bigger the bending loss.Bending loss can be used to measure the temperature.