A dissolved oxygen sensor is reported based on plastic optical fiber as sensor head and light transmit element. In order to enhance the systems sensitivity, the sensor head is bent into biconical-shape, and dissolved oxygen concentration is measured based on fluorescence quenching phenomenon. Dichlorotris (1, 10-phenanthroline) ruthenium (II) is used as fluorescence indicator and Sol-gel method is used to prepare the sensor material. According to Stern-Volmer equation, the dissolved oxygen concentration is linear with fluorescence intensity. We measure the relationship between dissolved oxygen concentration and fluorescence intensity and find that they are sub-linearity. In order to explain this phenomenon, we propose a model. By conic fitting the measured curve, the conclusion is drawn that 85% sensitivity film is not affected by dissolved oxygen.