According to the difference between bending loss when in air and liquid, the oil annunciator which is used to detect the state of oil level is proposed. By analyzing the characteristics of the sideemitting optical fiber and the detection principle of the optical fiber oil annunciator, the circuitry and mechanical mechanism are designed. Finally, the experiment of principle is tested. And by changing light intensity and temperature in the environment, attributes and temperature in the liquid to test performance of the annunciator, the results show that the annunciator’s accuracy and hysteresis range are less than 1 mm and the annunciator is stable. The results also prove that the annunciator is feasible.