Abstract: In response to the difficulty in evaluating the transition zone size of aeroengine disk with complex grain size distributions, this paper proposed a method of using ultrasonic testing to evaluate their transition zone boundaries. Firstly, ultrasonic C-scan was used to detect the hybrid disk of aeroengine; Then the C-scan results was preprocessed through mean filtering; Finally, the boundary recognition algorithm proposed in this paper was used to extract the boundary of the transition zone from the C-scan results, thereby achieving quantitative evaluation of the size and position of the boundary of the transition zone in the hybrid disk. The results showed that the proposed boundary recognition method improved the accuracy and efficiency of boundary evaluation in the transition zone of hybrid disk.