A novel fiber optic sensor used for temperature measurement under the offshore oil well was proposed based on the displacement of the optical absorption edge occurring in semiconductors under the influence of temperature variation as a result of temperature induced energy shifting of conduction band extrema. The structure of the sensor and the measurement principle were described. The common path reference measurement and node type error compensation technologies were developed. And the detailed theoretical analysis indicated that the proposed sensor system could effectively improve measurement errors caused by the light fluctuation, difference and variation of the detector responsiveness and the circuit magnification times, the transmission loss, and so on. It approved that this sensor system could be applied under the long term formidable conditions with fairly good measurement stability. The temperature measurement range is from -20℃ to 175℃.