A theoretical model is proposed and a novel system is built for measuring temperature based on the change rule for output power of single-mode-multimode-single-mode (SMS) fiber with temperature. The new measuring system can get the variation of the temperature which has a linear relationship with the output power by measuring the phase change of the output power. The calibration coefficient between the phase and the temperature is 2.5347 for a particular SMS fiber by comparing the two temperature variation. Then, a naturally rapid warming and a linearly slow warming of the temperature are checked by the SMS fiber and a thermometer. It is shown that the error is less than 5%.