Defects may sometimes develop within the interface of a diffusion bonded dual-alloy blisk and the ultrasonic evaluation is the best method of controlling the interface defects. However, the acoustic impedance difference and complex geometry bring great difficulty to the accuracy and reliability of ultrasonic evaluation. An evaluation method is proposed in this paper, in which the ultrasonic signal is introduced into the inner hole wall of the blisk with a 90° acoustic mirror, realizing the ultrasonic evaluation of bonding interface of dual-alloy blisk. Experiments are conducted to study the evaluation capability of the proposed method for bonding interface of dual-alloy blisk in terms of sensitivity and SNR, etc. Results show that the evaluation sensitivity of the proposed method can reach a flat-bottomed hole equivalent of Ф2.0 mm, and the ultrasonic evaluation results are in good agreement with those of metallographic analysis, verifying the reliability of the proposed method.