Terahertz (THz) technologies have broad application prospects in ultrafast space communication, heterodyne detection, biological detection, non-destructive testing and national security. Ultrafast THz detectors, which can respond to the THz light with modulation rate larger than 1 GHz, are the key component of fast imaging, space communication, ultrafast spectroscopy and THz heterodyne applications. Theoretically, the traditional THz detectors based on heat effects are difficult to meet the requirements for fast detections, while the semiconductor based THz detectors can work under the condition of ultrafast detection. Photoconductive antennas with ultrafast response time are suitable for room-temperature broad-spectrum THz detections. Schottky barrier diodes, superconductor-insulator-superconductor mixers and hot electron bolometers are promising candidates for high-speed THz spatial heterodyne and direct detections attributable to their high conversion efficiency and low noise. High-mobility field effect transistors based on two-dimensional graphene material have the advantages of high sensitivity and low impedance, which make this kind of device have great potential applications in room-temperature high-speed detections. THz quantum well detectors (THz QWPs) based on inter-subband transitions are very suitable for the applications in high-frequency and high-speed detections because of the advantages of high responsivity, small value and integrated packaging. Recently, we have demonstrated 6.2 GHz bandwidth modulation by using THz QWPs, the fast THz receiving device. On the other hand, low working temperature and low coupling efficiency are the main factors that restrict the applications of THz QWPs. From the Brewster angle, 45 polished facet coupling structure, to one-or two-dimensional metal grating and surface Plasmon polariton coupling configuration, researchers often explore the appropriate coupling mechanism which can not only couple the normal incidence THz light, but also improve the coupling efficiency substantially. The sub-wavelength double-metal micro-cavity array coupling structure has two advantages which make THz QWPs a key candidate for fast imaging and detection in THz band:firstly, the patch antennas on the device surface can effectively increase the light absorption region, and the periodic structure can make the normal incidence THz light fulfill the rule of intersubband transition. Secondly, the sub-wavelength size double metal structure can restrict the light within a very small volume, and the electric current will be enhanced by the resonance effect when the cavity mode is equal to the peak response frequency, which can suppress the dark current and improve the optical coupling efficiency of the device. In this paper, several ultrafast THz detectors are reviewed and the advantages and disadvantages of various detectors are also analyzed.