Terahertz wave has non-destructive nature and fingerprint characteristics for a large number of biomolecules, thus has a good application prospect in the field of medical imaging. In this review, we presented a brief introduction on the terahertz medical imaging systems, and the applications of terahertz medical imaging in biological tissues from in vitro to in vivo. Terahertz wave can be strongly absorbed by water, then the terahertz imaging contrast will be severely deteriorated in vivo. So the terahertz medical imaging was mainly used for detecting epidermal tissues or biological tissues with pretreatments, including excision, dehydration and so on. This review also concluded the recent development of nanoparticle contrast agents for improving the contrast of terahertz imaging in vivo. Finally, the future development of terahertz medical imaging was predicted.