The invention relates to an automatic ultrasonic detecting method and device of defects inside an old crank of an engine, belonging to the field of automatic ultrasonic detection. The method comprise the steps of: firstly, calibrating a single crystal angle probe by adopting a crank probe calibration test block, determining detection parameters of a real front rake angle beta and a real side rakeangle beta' of the probe, a probe zero point, material sound velocity of the crank, and the like; and then detecting the crank by adopting a crank automatic ultrasonic detection device, wherein the detection device comprises a crank clamp and an ultrasonic probe clamp, the crank clamp is used for clamping the crank and enabling the crank to rotate at constant speed, the ultrasonic probe clamp is used for clamping the probe, and after the crank rotates for one circle, the probe can check a shaft neck for one circle. Internal defects of the shaft neck and an R angle can be automatically detected by adopting one probe, thus the reliability and the detection efficiency are furthest improved. The method can be used for detecting the defects of cracks, impurities, air holes and the like with equivalent size of above 1mm inside the shaft neck and at the R angle.