To develop a compact laser with high power as well as good beam quality，a composite Nd∶YAG/YAG transparent ceramic polygonal active mirror gain medium structure is reported，and conceptual validation is conducted by researching the laser output characteristics. The composite Nd∶YAG/YAG transparent ceramics used in this work are fabricated by the transparent ceramic technique. The five sides of the polygonal active mirror are Nd∶YAG doped regions，which are also laser gain mediums. The optical properties of the composite Nd∶YAG/YAG ceramics and the laser performances of polygonal active mirror are investigated. With a pump power of 4.05 W，a maximum output power of 0.43 W is obtained，and the corresponding optical-to-optical and slope efficiencies are 10.6% and 16.9%，respectively. The corresponding beam quality factors are $M2X=1.21$ and $M2Y=1.24.$ The results of this study demonstrate the feasibility of using transparent ceramic techniques to fabricate laser gain mediums with functional structures，which brings more flexibility to the design of lasers.