A detection system of ultrasonic stress wave based on fiber Bragg grating was established, whose working principle was expressed. The linear relationship between the output voltage and plate strain based on the connection between the dynamic strain and center wavelength shift of fiber Bragg grating was derived. A pulse of 28 kHz generated by ultrasonic signal generator drived the ultrasound vibrator, the signal was conveyed to a 5052 aluminum alloy plate through a organic glass wedge to generate stress waves which were detected by a fiber Bragg grating strain sensor. After that, a new wave packet appeared in the measured ultrasonic waves when there was a crack of 98 mm in the plate. The arriving time difference between the new wave packet before and after the damage can be used to determine the crack location. The position deviation of the crack is 2.7 mm. It is shown that instead of an ultrasound probe, the fiber Bragg grating can be used to detect the low-frequency ultrasound waves.