As core technology of the third industrial revolution, 3D printing or additive manufacturing is a novel method of making parts directly from digital model in the approach of layer-by-layer material build-up. This mould-free production method could be used to produce fully dense high precision metal parts in short time. As 3D printing featured in free part design, part complexity, lightweight, part consolidation and function design, its application has received special attention in for the fields of aerospace, oil & gas, marine, automobile, mould making and medical treatment. Firstly, basic principle, characteristics and classification of metal 3D printing technology were briefly introduced. Then several kinds of metal 3D printing technologies were emphatically introduced, which covered selective laser sintering (SLS), selective laser melting (SLM), direct metal laser sintering (DMLS), electron beam molding (EBM) and laser engineered net shaping (LENS), including basic principles, advantages and disadvantages as well as specific application field of the technologies. Finally, the advantages, main problems at present and future development trends of metal 3D printing technology were summarized and prospected.