In the current stage, conventional silicon-based devices are suffering from the scaling limits and the Fermi level pinning effect. Therefore, looking for low-resistance metal contacts for semiconductors has become one of the most important topics, and two-dimensional (2D) metal/semiconductor contacts turn out to be highly interesting. Alternatively, the Schottky barrier and the tunneling barrier impede their practical applications. In this work, we propose a new strategy for reducing the contact potential barrier by constructing a donor-acceptor heterostructure, that is, Ca2N/MoS2 with Ca2N being a 2D electrene material with a significantly small work function and a rather high carrier concentration. The quasi-bond interaction of the heterostructure avoids the formation of a Fermi level pinning effect and gives rise to high tunneling probability. An excellent n-type Ohmic contact form between Ca2N and MoS2 monolayers, with a 100% tunneling probability and a perfect linear I-V curve, and large lateral band bending also demonstrates the good performance of the contact. We verify a fascinating phenomenon that Ca2N can trigger the phase transition of MoS2 from 2H to 1T'. In addition, we also identify that Ohmic contacts can be formed between Ca2N and other 2D transition metal dichalcogenides (TMDCs), including WS2, MoSe2, WSe2, and MoTe2.