The directivity of ultrasonic stress transducer has an important influence on the accuracy of ultrasonic stress detection. In this paper, the semi-cylindrical block method is proposed. The transducer to be calibrated radiates sound waves to the interior of the semi-cylindrical block, and the receiving transducer receives the signals at various angles on the cylindrical surface of the block, and the directivity of the calibrated transducer is obtained by analysis. The directivity of the longitudinal wave transducer is calibrated by the semi-cylindrical block method, and its -3 dB beam width is 5.01°, the relative deviations from the measured value and theoretical value of the hydrophone method are 3.09% and 2.66%, respectively. The directivity of the critical refracted longitudinal wave transducer is obtained by the semi-cylindrical block method, and its -3 dB beam width is 10.55° and the relative deviation from the simulated value is 2.23%. The semi-cylindrical block method is more in line with the actual working conditions of ultrasonic stress, and is not limited by the water tightness of the transducer and the operating principle of the transducer. It is suitable for any type of ultrasonic stress transducer, and the calibration is more efficient and fast.