In order to achieve real-time monitoring of the temperature of the downhole submersible electric pump unit, a Mach-Zehnder interferometer (MZI) high-temperature fiber optic sensor composed of coarse cone singlemode-multimode-singlemode (CCSMS) was designed. The structure adopts a direct fusion method to fuse the single-mode fibers with the multi-mode fibers. Then, by adjusting the fusion parameters of the fusion splicer, a thick cone structure is fabricated on the single-mode fibers. Finally, the prepared structure is embedded in the U-shaped groove of the copper substrate to achieve the sensitization packaging of the sensor. The temperature response characteristics of the packaged sensor were tested. The experimental results show that the sensor achieves a temperature sensing sensitivity of 124.9 pm/℃ in the temperature range of 40℃ to 250℃. The stability and repeatability of the sensor were tested. The results show that the maximum stability error of the sensor is about 0.44%, and the maximum repeatability error is about 2.29 pm/℃. The sensor has the characteristics of high sensitivity, good repeatability and stability, and is expected to be used for the temperature of the electric submersible pump unit in oil and gas wells.