Abstract: In order to study the influence of stress level on ultrasonic wave velocity of concrete, plain concrete axial tensile specimens, plain concrete axial compressive specimens, plain concrete cube compressive specimens and reinforced concrete flexural members were designed and poured in this paper. The ultrasonic wave velocity tests of plain concrete specimens and reinforced concrete members under different stress levels were carried out respectively. The results show that the tensile and compressive stress levels have little effect on ultrasonic wave velocity of concrete specimens, which can be ignored. The low stress level has little effect on ultrasonic wave velocity of reinforced concrete members, and the high stress level has a great influence on ultrasonic wave velocity when cracks appear in concrete members. The tensile stress-ultrasonic wave velocity relationship curves of concrete members can be divided into four stages, including no damage period, damage development period, damage stability period and component failure period. When the tensile stress reaches 15%～25% of ultimate tensile stress, the component enters damage development period from non-damage period, and the ultrasonic wave velocity does not change at this time. When the tensile stress reaches 35%～55% of ultimate tensile stress, the component enters damage stabilization period from damage development period, and the ultrasonic wave velocity is reduced by about 5%. When the tensile stress reaches 60%～75% of the ultimate tensile stress, the component enters failure period from damage stable period, and the ultrasonic wave velocity is reduced by about 8%.

Key words: concrete, tensile stress level, compressive stress level, ultrasonic, non-destructive testing, crack