In this paper, a theater design tool, “Auto-theater”, tentatively made by the author is proposed to improve the theater design based on the parameter design. In the past, almost all the architects used the tools made by the software manufactures to draw graphics. Now, a few architects start to make the tools by themselves to generate designs. The primary objective is to help the architects focus themselves from using drawing tools onto making their own design tools, so that they will better understand the architectural rules. The paper describes in detail the development of “Auto-theater”, discusses its application and outlooks its future.