<p>Compressors in helicopter and unmanned aerial vehicle engines serve as essential components in the air compression system, ensuring engine power and efficiency across variable altitude environments. Deep understanding of the aerodynamic loss mechanisms of high-altitude compressors are the foundation for achieving high-performance compressor design. To investigate the energy-loss mechanisms of compressors at different altitudes, a new local-loss-coefficient (LLC) based on entropy generation theory is proposed. Compared with the traditional entropy generation rate method, LLC considers the gas compressibility in derivations, making it suitable for the high-speed compressors. Additionally, it introduces the concept of local power to eliminate the influences of varying boundary conditions of compressor on the energy-loss quantifications, making it applicable for different altitudes. Based on the LLC analyses, the common energy-loss mechanisms of both sea level and high-altitude impellers are drawn. The energy-loss of impeller leading-edge is mainly due to the mixing of TLF (tip leakage flow) and MF (main flow), while the energy-loss of impeller downstream region is caused by the interaction of TPSF (tip passage secondary flow) and TLF. The mechanisms of the additional energy-loss due to the increased altitude are also drawn. The high-altitude compressor possesses higher pressure gradients inside the impeller, this causes strong interactions between TLF and MF, as well as interactions between TLF and TPSF, leading to greater internal energy-loss.</p>