Wide field-of-view (FOV) and high-resolution is one of the goals of optical microscopy. However, limited by the optical design in traditional optical microscopes, the space bandwidth product (SBP) is generally in the order of megapixels, and thus, high-resolution and wide FOV cannot be achieved at the same time. On the other hand, complex optical systems have also made microscopes increasingly expensive, cumbersome, complex and difficult to maintain, greatly limiting their promotion and application. Lensfree on-chip microscopy is a new computational imaging technology: without the imaging lens to focus, the sample is directly attached to the imaging sensor to record the diffraction patterns and the object information can be achieved with the corresponding reconstructed method. Due to its wide FOV, high-resolution, label-free detection, low-cost, perfect portability and three-dimensional (3D) imaging, the lensfree on-chip microscope is expected to expand the boundaries of traditional microscopic imaging technology and becomes a new type of fast, point-of-care testing (POCT) tool. In this paper, a review was given to introduce the basic principles, experimental systems, reconstruction methods and applications of lens-free imaging. Finally, the changeling problems as well as future research directions were also discussed.