Abstract: Micro-mixer has great application potentials in many fields such as material synthesis, pharmaceutical preparation and biochemical detection due to its advantages of saving reagents, higher mixing index and easy integration. In order to further improve the mixing performance, to ensure the safety of the mixing process and the accuracy of the biochemical reaction results, a new square wave micro-mixer with extended cavity was designed. Under the premise of considering the mixing index and pressure drop, the effects of the width of slit, the length of slit, and the height of extended cavity on the mixing performance of micro-mixer were analyzed by experiment and simulation. The optimal structural parameters were achieved under different Reynolds number ($\mathit{Re}=20$). Compared with the square-wave micro-mixer, the mixing strength of the square-wave micro-mixer with the extended cavity is higher when $\mathit{Re}$. Moreover, the gap of mixing index between two square-wave micro-mixer reaches maximum, up to 12%, at $\mathit{Re}<10$. Under the same Reynolds number, the pressure drop of the square-wave micro-mixer with the extended cavity is lower than that of the square-wave micro-mixer. Meanwhile, the analysis for the internal flow field of the square-wave micro-mixer with the extended cavity was carried out. It is found that the eddy current is introduced on the basis of the laminar flow state of the fluid because of the existence of the extended cavity structure, which means the change of the flow state of the fluid in the channel and the enhancement of convection effect, thus the mixing performance is further improved.