Several evaluation methods for laser beam quality were reviewed,their applicability for the outline characteristic of slab laser was analyzed and an evaluation method for slab laser was chosen. To get good beam quality of slab laser,a correlative technique can be used to improve beam quality of slab laser while the laser is designed. When the shape of the rectangle beam of slab laser is changed,the adaptive optics technique can be used to improve beam quality. For output beam of the practical slab laser,good quality beam can be got after adaptive optics is used.