We propose and demonstrate an intensity-demodulated distributed Bragg reflector (DBR) fiber-laser sensor for ultrasonic detection. The DBR laser is formed by two distinctively different fiber Bragg gratings (FBGs) written on an erbium-doped fiber. One of the FBGs is short but relatively strong with sharp spectral edges and is used as the sensing element; the other one is long but relatively weak, leading to a narrow reflection bandwidth that sets the lasing wavelength on the spectral slope of the sensing FBG. The acoustic signal causes spectral shifts of the sensing FBG, which consequently modulates the cold cavity loss of the DBR fiber laser and results in laser intensity variations in response to the cold cavity loss variations. Therefore, the ultrasonic signal can be directly detected from the laser intensity variations. The intensity demodulation method leads to a simple-structure and low cost ultrasonic sensor system.