Damages such as cracking or impact loading in civil, aerospace, and mechanical structures generate transient ultrasonic waves, which can be used to reveal the structural health condition. Hence, it is necessary to find a practical tool based on ultrasonic detection for structural health monitoring. In this work, we describe an intelligent fiber-optic ultrasonic sensing system, which is designed based on a fiber Bragg grating (FBG) and a reflective semiconductor optical amplifier (RSOA) used as an adaptive source, and demodulated by an adaptive photorefractive two wave mixing (TWM) technique without any active compensation of quasi-static strains and temperature. As the wavelength of the FBG shifts due to the excited ultrasonic waves, the wavelength of the optical output from the fiber cavity laser shifts accordingly. With regard to the shift of the FBG reflective spectrum, the adaptivity of the RSOA-based laser is analyzed theoretically and verified by the TWM demodulator. Additionally, due to the response time of the photorefractive crystal, the TWM demodulator is insensitive to low frequency-FBG spectral shift. The results demonstrate that this proposed FBG ultrasonic sensing system has high sensitivity and can respond the ultrasonic waves into the megahertz frequency range, which shows a potential for acoustic emission detection in practical applications.