The real-time detection of scene echoes is a prerequisite for beam synchronization in spaceborne/airborne hybrid bistatic synthetic aperture radar （SAR）. To address the issues of weak scene echoes， range migration， and energy dispersion in hybrid bistatic SAR， a frequency-domain M/N detection method based on the analysis of scene scattering characteristics and echo models is proposed. The method leverages the correlation of adjacent PRF scenes to aggregate scene energy， effectively mitigating the impact of range migration. Simulation and experimental results demonstrate that the proposed algorithm achieves a high detection probability and superior performance compared to conventional detection algorithms. In addition， it offers strong resilience to jamming， low computational complexity， and is suitable for real-time implementation， making it practical for engineering applications.