Miller cycle and high compression ratio technology are the advanced technologies of spark ignition engine in hybrid vehicle at home and abroad. The influences of Miller cycle and high compression ratio on fuel economy and their correlation were studied with the model of simulating working process for a 1.2L 3-cylinder turbocharged GDI engine. The results showed that the single use of Miller cycle technology cannot improve fuel economy and the single use of high compression ratio can improve fuel economy under certain conditions but will cause severe knock. Miller cycle and high compression ratio technology have a strong correlation in suppressing knock, therefore, it is necessary to adopt Miller cycle to suppress knock while improving fuel economy by increasing geometric compression ratio.