In the planetary automatic transmission design, efficiency is one of the primary performance indicators determining the final selection from multiple candidates. However, efficiency evaluation is the last step of conceptual design, thereby causing the trial result is often known at the end of the design. A new methodology based on geometry is presented for multistage planetary automatic transmission design, ranging from clutching sequence synthesis to efficiency evaluation. The emphasis is placed on a unified model for design steps with different analysis principles to predict power transfer characteristics earlier. Some new results and improvements are also presented, such as general speed ratio (SR) change law, simultaneous traversal of different shift types, and the efficiency formula. An example is provided to illustrate the applicability to a 3-DOF planetary automatic transmission and shows that early identification of power transfer characteristics can lock in necessary subsequent calculations, thereby eliminating unnecessary analysis and speeding up the design process.