Terahertz (THz) waves have a good transmissivity on non-polar materials and no ionization effects on biomedical tissues. Therefore it is ideal for the applications such as non-destructive testing and biomedical imaging. The imaging system based on THz quantum well photodetectors (THz QWPs) has higher imaging resolution, faster imaging speed, higher signal-to-noise ratio (SNR), and more compact structure as the THz QWPs have fast response, high responsivity, low noise equivalent power, and tiny size. This paper reviews the research progress of the imaging system based on THz QWPs. The direct transmission and direct reflection imaging systems have simple light paths, and the 3D imaging system can obtain 3D information of objects. However, the imaging speed, the resolution and SNR are low. The archimedean spiral scanning imaging system progressed in imaging speed, but the resolution is still not high. The confocal scanning imaging system has a short imaging time and a relatively high imaging resolution, nevertheless, the SNR is low. The pixel-less imaging system has a diffraction-limited resolution, extremely short imaging time, and high SNR, is the most promising one above. There are some tips for system performance improvement. First, the imaging resolution can be optimized by the optical confocal methods. Second, the large imaging area can be achieved by optimized optical path design. Third, high imaging speed can be achieved with no mechanical stop scanning, multi-pixels detectors, or reducing signal acquisition time with an ultrafast detector. Forth, the SNR is mainly relying on the steady optical path, optical source power, and detector sensitivity. However, these factors are always competitive, a trade-off must be made to achieve an optimized imaging solution for a specific application. It improves the light output stability and beam quality with a more stable fixture for the source and the detector mounting. The improvement of the detector response speed, detection sensitivity, and array size are also working. It is believed that the THz imaging will become faster (real-time/ultra-fast), more accurate (higher resolution), and simpler (lower systems complexity) with these efforts. And it will play an important role in biomedical and industrial imaging in the future.