Possessing high resolution and rich information, optical imaging is one of the most important techniques for people to obtain information. Photons are information carriers in optical imaging systems. The high-quality reconstruction of optical image depends on the efficient coupling of signal photons and the accurate decoupling of optical information. However, in important application scenarios such as remote sensing or biological imaging, due to the long operating distance or low radiation power, the number of signal photons from the object to the detection plane is small, and the signal-to-noise ratio is low, thus bringing great difficulties to the design of optical system, the signal detection and the image reconstruction, and seriously limiting the performance of optical imaging. How to obtain high-quality images under extremely weak light conditions is not only a basic problem of photoelectric imaging system research, but also a key technology to promote the vigorous development of optical imaging with a larger field of view, longer working distance and higher information flux. In recent years, with the support of light field modulation and quantum detection technology, and based on the high-order classical/quantum correlation of light field, ghost imaging has brought new opportunities for the development of optical imaging technology under extremely weak light conditions, due to its high detection sensitivity and strong ability against interference. This paper briefly reviewed the principle and mechanism of ghost imaging, and systematically introduced the schemes and methods of ghost imaging under very weak light conditions. The physical essence of these methods from the level of photon dynamics was introduced, the capability limits of these methods were discussed, and the applicable scenarios of these methods were compared.