Based on a 1.5T gasoline direct injection（GDI） hybrid engine with high compression ratio, the effects of engine cooling system design on knock and performance were studied by establishing a computational fluid dynamics（CFD）model and bench test analysis. The system adopted a series cooling mode of cylinder head first and then cylinder block. By optimized design of the cylinder block and cylinder head water jacket, efficient heat exchange was achieved in the exhaust nose bridge area, which could effectively suppress knock. By optimized design of electronic water pumps, EGR coolers and watercooled intercoolers, the system met the cooling requirements under high EGR rates while featuring a compact size and low pressure drop. This resulted in a 0.6% increase in the engine maximum effective thermal efficiency and a 11% increase in maximum net power without enrichment. Furthermore, introducing an additional 5% of EGR gas could boost maximum power by another 8%.