Fiber-optic ultrasonic sensors possess the ability to detect ultrasonic waves by recovery of light intensity, wavelength, phase, and polarization. Compared with traditional electrical ultrasonic transducers, fiber-optic ultrasonic sensors have several merits, such as broadband response, high sensitivity, disturbance resistance, and good reusability, which are helpful to improve the reliability and efficiency of ultrasonic detection in underwater defense security, bioimaging, nondestructive inspection, and imaging of seismic physical models. To date, according to the principle, fiber-optic ultrasonic sensors can be classified into three types, including intensity modulation, fiber-optic interferometers and fiber gratings. For the intensity-modulated fiber-optic ultrasonic sensors, ultrasonic waves can be detected by measuring optical fiber coupling loss, fiber transmission-reflection loss, fiber reflection loss and fiber polarization loss. The phase difference in fiber-optic interferometers can be modulated by ultrasonic strain. According to the interference mechanism, fiber-optic interferometric ultrasonic sensors are generally based on Mach-Zehnder interference, Fabry-Perot interference, Michelson interference and Sagnac interference. For the ultrasonic sensors based on fiber gratings, the grating length is supposed to be shorter than the ultrasonic wavelength so that the ultrasonic stress presents constant along the fiber gratings. Currently, the approaches of spectral edge filtering and wavelength-matched filtering are utilized to transform optical signals into voltage signals, which highly depend on the slope of the grating spectra. Thus, the fiber gratings with extremely narrow 3-dB bandwidth, such as phase shifted fiber Bragg grating, are preferred for highly sensitive ultrasonic detection. Besides the fiber-optic passive sensing, the distributed feedback fiber laser and distributed Bragg reflector also exhibit outstanding advantages in ultrasonic detection. Fiber-optic ultrasonic detecting technique is one of the hot topics in international research community, which is an effective method to evaluate the microstructure and related mechanical properties, and detect the microcosmic and macroscopic discontinuities of solid materials. In this paper, three typical applications of ultrasonic detection, i.e., monitoring of smart structure and health, biomedical imaging, and imaging of seismic physical models are reviewed, respectively. Our group has been engaged in the research fields of fiber-optic geophones and ultrasonic sensors for seismic exploration for decades. Several fiber-optic ultrasonic sensors with smart packaging are proposed and also used for the scanning imaging of two physical models. In this paper we review the sensing mechanism, fabrication method, and current status of three types of fiber-optic ultrasonic sensors, respectively. Besides, the corresponding applications and technology challenges are also summarized. In particular, we present several kinds of home-made optical fiber ultrasonic sensors as a new technology applied in the imaging of seismic physical models. Overall, after decades of efforts, gratifying achievements have been achieved in the research of fiber-optic ultrasonic sensors. Further work needs to solve various technical issues, such as sensitivity, stability, structural microminiaturization, and multiplexing, etc. The next step will focus on the research issues in ultrasonic detection of seismic physical models, performance improvement, and multiplexing technology for distributed sensing. Miniaturization of fiber sensors and instrumentation of sensing system will also be the important research topic. The final objective of the research is to build a well integrated fiber-optic ultrasonic detecting system with high sensitivity and stability, networking construction, and proprietary intellectual property rights.