Abstract: This paper describes a new method for aligning a positive-branch co -focal unstable resonator without any precise alignment instrument. It also eliminates a small hole in centre of the concave mirror. By utlizing the reflectivity of the backside of the outcoupling mirror and a converging beam with definite focal length and aperture, we can successfully align an unstable resonator. Meanwhile, a bright coaxis reference beam is provided which can be used to adjust outside optical elements. The alignment accuracy is estimated to be about 10μrad. Furthermore, the experiment demonstrated that the zero-grade reflected beam can be filtered successfully when the method presented in this paper was used. Therefore, He -Ne reference beam can show the sitiuation of the alignment and parameter variation of the resonator more clearly. After a concave mirror with a small hole is replaced by a concave mirror with no small hole on it, the output performance of a DF laser is improved.