A temperature sensor based on polymer microsphere cavity is proposed, which measures the change of external temperature through the resonant wavelength shift and demonstrates the characteristics of high compactness and sensitivity. The finite-difference time-domain method is firstly employed to simulate the structure of tapered fiber coupled polymethyl methacrylate (PMMA) microspheres to verify its feasibility of whispering gallery mode excitation. Experimental results show that whispering gallery mode with a quality-factor on the order of 10⁴ can be excited by evanescently coupling a polymer microsphere with a diameter of tens of micrometers through a tapered fiber with a diameter of 1.8 μm. Packaging the device by combining spot and complete coating can keep a stable coupling state between the tapered fiber and the microsphere and protect them from external contaminant. Red shift happens in the resonant spectrum of the microsphere cavity as the external temperature decreases since its negative thermo-optic coefficient is larger than the thermal expansion coefficient. When the external temperature varies in the range of 20-30 ℃, the polymer microsphere demonstrates a sensitivity of 68 pm/℃. In comparison with the conventional optical fiber sensors, lower detection limit can be achieved by the proposed temperature sensor with a higher quality-factor, which can be potentially used in the in-situ temperature precise measurement in a limited space.