A double-layer micromixer made from Polydimethylsiloxanes(PDMS) was designed to realize uniform mixing for microlitre liquids.Its fabrication methods and the effects of geometrical sizes and Reynolds numbers(Re) on it were investigated.The design principles of the micromixer with main-assist channels were presented based on Fick’s first law and the velocity and concentration distribution of the micromixer in different geometrical sizes and Re were simulated by finite element method.The calculated results demonstrate that the mixing efficiency increases with the decreases of Re and width ratio and the increase of channel length.Then a mixer with a channel length of 9 mm, depth ratio of 0.71 and a width ratio of 1 was fabricated.The results of mixing experiments of water and red ink show that the micromixer designed at Re<5 can realize the fast mixture of two streams and the efficiency of mixture increases with the decrease of Re.It can meet the requirements of micro fluid’s rapid mixing at low Re.