Here, the concentration of TNT was detected by using evanescent absorption method, U-shaped plastic optical fiber was used as sensor head and light transmit element. A high brightness blue light-emitting diode with spectrum centered at 470 nm was used as excitation source. The relationship between the transmitted light power of U-shaped sensor head and TNT concentration was linearly and the linear coefficient was 0.944. The system showed excellent reversibility when the sensor tested by alternately cycling between 0 and 25 mg/100 ml TNT solution in five cycles. The stability of the sensor head in 25 mg/100 ml TNT solution was also tested within two hours. The proposed sensor is a cost-effective alternative to traditional TNT sensors and provides a platform for other optically based sensors.