A micro ultrasonic sensor based on an advanced suspended-core fiber is proposed and employed for in-lab seismic physical modeling. A free suspended core is obtained by acid corrosion and two cascaded uniform fiber Bragg gratings (FBGs) are imprinted in the suspended-core fiber. The sensor response and stability are largely improved due to the using of dual-FBG reflectors instead of weak-reflection fiber mirrors for constructing an in-fiber interferometer. The characteristics of reflection spectra and ultrasonic response of the sensor are analyzed and demonstrated experimentally. Comparative measurements are also carried out to prove the sensor superiority over the conventional weak-reflection one. Moreover, the sensor is used for seismic physical modeling to show its ability of practicable usage. Both the crosswell seismic and surface seismic in seismic exploration are modeled respectively based on reservoir and fault models. Various reservoir velocities are measured and each is consistent with the reported results. The fault features are also well reconstructed in the form of a cross-section model image. The improved sensor approach greatly promotes the application of the suspended-core fiber for weak acoustic detection in seismic physical modeling.