In this paper, a kind of temperature sensor which can detect a small-area heat source with high sensitivity is designed by using the property of different thermal expansion coefficients of materials. The temperature sensitive element of the sensor is a silicon nitride cantilever beam which is coated with metal on its upper surface. Due to the difference of thermal expansion coefficients between the metal and silicon nitride, the cantilever beam will bend in the direction of rapid change of the temperature gradient, and the bending amount will be positively correlated with the temperature when the ambient temperature of the cantilever beam changes. In the experiment, the bending amount of the beam is measured by the optical lever, and the relationship between the temperature and the output voltage of the detector is established by calibration. The results show that the sensitivity of the sensor can reach 4.86 mV/℃ and the temperature resolution can reach 0.04 ℃. In order to verify the applicability of the sensor for measuring the small-area heat source, the heat generated by heat sources of different areas is measured depending on the calorific property of NaYF4 under laser excitation. The results show that it still can be measured even the heatin area is only 0.07 mm2 and the accurate measurement for temperature of the small-area heat source can be realized.