Aimed at the problem of ultrasonic axis’s sound pressure impacting on flat bottom hole defect echo wave,this paper studies the approach of ultrasonic flaw based equivalent method.Firstly,according to the sound pressure model from the point sound source to a point on the sound axis,the transmitting sound field of wave axis has been set up and echo sound pressure model of the flat bottom hole reflector has been established with Fresnel approximation theory.Then the effects of the dimension of flat bottom hole,the depth of flat bottom hole,the sound source size and the transmission frequency on the echo wave of the flat bottom hole have been analyzed theoretically and simulated with experiment.The results showed that the model could be a guide to the quantitative nondestructive assessment.