Quantum circuits are the tools of realizing the unitary evolution of quantum state, and they are mainly made up of one-bit and two-bit quantum gates. Barenco proposed a method for constructing n-bit quantum gate by using basic two-bit quantum gates in 1995. Zhang proposed the improved method for constructing n-bit quantum gate by using basic two-bit quantum gates in 2001. The method for constructing n-bit quantum gate by using basic two-bit quantum gates is proposed by improving the two methods mentioned. The scheme is more simple and easier to implement and the incorrect conclusion by Zhang is amended.