A dissolved oxygen(DO) sensor system based on fluorescence quenching method is reported,where the plastic optical fiber is used as sensor head and light transmitting element while dichlorotris(1,10-phenanthroline) ruthenium as fluorescence indictor.Fluorescence lifetime was measured to detect the DO concentration;the lifetime of fluorescence indictor is detected by phase shifting method.Three different sensor heads(u-shaped,double taper and spiral-shaped) are fabricated.Experimental results show that the spiral-shaped sensor head is more sensitive than the u-shaped and the double conical sensor heads;the fluorescence indictor concentration also plays a very important role in the sensitivity of the system.A proper concentration of fluorescence indictor must be chosen to achieve high sensitivity.