An aromatic explosives sensor system based on fluorescence quenching method is reported. MEH-PPV is used as fluorescence indictor and plastic optical fiber as sensor head and light transmitting element. Fluorescence lifetime of fluorescence indictor measured by phase shifting method is used to detect the explosives concentration. The effect of fluorescence indictor concentration on system sensitivity is measured, and we find that when MEH-PPV concentration is 10 mg/L the system sensitivity reaches the highest point. We also measure the system sensitivity under different shaped sensor heads (U-shaped, biconical-shaped, spiral-shaped), and it is found that biconical-shaped is most suitable for the sensing system. The stability of MEH-PPV is studied, and the experimental results indicate that the bleaching rate of fluorescence indictor will get faster under the exposure of daylight.