Magnetically coupled wireless power transfer (MC-WPT) technology has garnered increasing research attention due to its rapid development and expanding application demands. Constant power output is crucial for energy transfer stability and operational safety. However, conventional autonomous MC-WPT systems for constant power output exhibit significant limitations. For instance, parity-time (PT) symmetric MC-WPT systems achieve stable power delivery solely within the strong-coupling regime, while under weak-coupling conditions, their output power becomes highly sensitive to load variations and detuning, leading to severe performance degradation. To address these challenges, this paper proposes an inverse design methodology for autonomous MC-WPT systems, which fundamentally diverges from traditional forward design paradigms. By systematically reconstructing generalized autonomous mathematical models with predefined constant-power objectives, we put forward an autonomous WPT architecture that demonstrates robust constant power characteristics. The proposed system exhibits exceptional immunity to load fluctuations and transfer distance variations while maintaining operational stability under detuned conditions. Experimental validation confirms the effectiveness of the methodology, offering a new idea for the research and development of WPT technology.