In this paper, a novel micromixer with complex 3D-shape inner units was put forward and fabricated by metal Additive Manufacturing (AM). The design of the micromixer combined the constraints of selective laser melting technology and the factors to improve mixing efficiency. Villermaux-Dushman reaction system and Compute Fluid Design (CFD) simulation were conducted to investigate the performance and the mechanism of this novel micromixer to improve mixing efficiency. The research found that the best mixing efficiency of this novel micromixer could be gained when the inner units divided fluid into five pieces with a uniform volume. Compared with a conventional micromixer without obstacle in the channel, the micromixer designed in this research achieved higher mixing efficiency and reduce the pressure drop by 10.34%. The mixing behaviour in this novel micromixer was discussed, which mainly contains two types: collisions and swirls. Via collisions, the fluid micro masses would hit each other directly, which broke the boundaries of micro masses and promoted the interchange of species in the whole flow field. In swirls, the fluid micro masses were drawn into thin and long slices, which increased the size of the contact area and enhanced molecule diffusion. ... mehrFinally, the application scheme of this novel micromixer was briefly discussed.