Increasing turbine load intensifies shock wave structures, leading to a significant rise in internal flow losses. Unveiling the mechanisms and patterns of flow losses induced by shock waves within high expansion ratio turbines is critical for turbine performance optimization. An entropy production based method was put forward for quantitatively identifying key flow structure losses in high expansion ratio turbines, and exploring the flow loss distribution and mechanisms of an in-house developed high-efficiency turbocharger turbine. The study reveals that secondary flow losses induced by shock waves account for 80% of total losses. The leakage flow and secondary flow structures are the dominated factors of loss in all operating conditions. The expansion ratio increases from 2.8 to 4.0, leakage flow losses decrease to 38%, while the proportion of shock wave losses sharply rises from 0.4% to 12.4%. Based on the above mentioned loss laws and mechanisms and combined with the response surface model of turbine performance, a parameterized model was constructed and parametric optimization design was achieved. The results show that the optimized turbine exhibits a significant reduction in shock wave intensity and induced losses, while the turbine efficiency improves 1.88%.