As a deeply precooled combined cycle engine, SABRE3 achieves deep integration of multiple cycles with different working medium, offering advantages such as a wide speed range and remarkable performance. However, the multi-cycle coupling configuration also introduces significant structural complexity and thermodynamic cycle interdependencies. To clarify the coupling mechanism between the closed helium cycle and the air cycle and enable efficient helium cycle design, this paper investigates a decoupling design method for closed helium cycle. The key lies in the characteristic parameter—cycle power-to-heat ratio (CPHR), defined as the ratio of mechanical power on Shaft 1 to precooler heat transfer power. By applying this characteristic parameter and transforming the constraints, theoretical expressions for the boundaries of the design feasible region are derived. The cycle power-to-heat ratio enables decoupling design for closed helium cycle. The feasible region represents the complete set of viable schemes for helium cycle design. For a given performance requirement, an optimal design scheme can be intuitively obtained through feasible-region-based cycle analysis. Results demonstrate that the design principle for minimizing hydrogen equivalent ratio is to maximize T23 and T24 within the feasible region.