Micromixer is a key component of various microfluidic systems, such as micro-total-analysis system and microreactors, which have wide applications for bioengineering analysis and chemical processing, etc. At microscopic scales, fluid mixing becomes difficult due to dominant viscous effects and laminar flow regime. In such a situation, the mixing is diffusion limited and slow. One important strategy to enhance micro fluid mixing is to produce chaotic advection. For passive mixer design, this is usually achieved through the perturbations enforced by the geometry of the flow channel. Chaotic advection raises the stretching and folding of the material interface, and so efficiently promotes the mixing. However, relevant studies also show that it is not easy to achieve the global mixing. For some mixers that have relatively regular spatial periodic structures, isolated/ stable regions may exist, where the fluids keep stubbornly concentrated and can not be dispersed into other flow regions. The existence of such isolated/ stable regions is a main obstacle for complete mixing and harms the efficiency of the device. Our study shows that, for such a partial chaotic mixer, its mixing can be improved through reorientation mechanism. That is, after a certain mixing length, we can modify the geometrical structure of the flow channel to induce the reorientation and reorganization of the fluids, i.e., to move the fluids from the stable region to the chaotic region. Using this way, the previously unmixed fluids can get mixed in the following section of the mixer. As a consequence, the global mixing can be achieved in the whole flow region.