In recent years, two-dimensional materials have received more and more attention in the development of semiconductor devices, and their practical applications in optoelectronic devices have also developed rapidly. However, there are still some factors that limit the performance of two-dimensional semiconductor material devices, and one of the most important is Ohmic contact. Here, we elaborate on a variety of approaches to achieve Ohmic contacts on two dimensional materials and reveal their physical mechanisms. For the work function mismatch problem, we summarize the comparison of barrier heights between different metals and 2D semiconductors. We also examine different methods to solve the problem of Fermi level pinning. For the novel 2D metal-semiconductor contact methods, we analyze their effects on reducing contact resistance from two different perspectives: homojunction and heterojunction. Finally, the challenges of 2D semiconductors in achieving Ohmic contacts are outlined.