The heat treatment of polymer bonded explosive PBX-03 with Φ20 mm×10 mm was carried out at 50 ℃,and was characterized by ultrasonic characteristic value. Some new information about the effect and mechanism of heat treatment on PBX-03 was obtained. The results show that the ultrasonic characteristic values of the specimens display a unified tendency during the heat treatment,and the adequate heat treatment is helpful to improve the internal quality of PBX-03,and the irreversible growth of TATB in PBX-03 is probably the main reason causing the results above.