Abstract: To improve fuel/air organization and combustion of spark ignition (SI) two-stroke piston engine with aviation heavy fuel, a direct injection (DI) combustion system was designed and studied based on CFD simulation for a SI two-stroke engine, and a combustion chamber design method was obtained suitable for the DI mode of aviation heavy fuel engine. Taking parametric design for the DI combustion chamber, the influence law of the main structural parameters on the fuel/air mixing process was obtained. The results show that variation of the offset distance, radius and depth of the chamber can affect the interaction among the three parts, cylinder airflow, spray and the chamber wall, so as to produce different fuel/air mixing effects. Through the orthogonal parameter analysis method, taking indicated power and indicated specific fuel consumption rate as evaluation parameters, the optimal combination of chamber parameters was obtained and met the performance requirements. The results show when the parameters set as follow, 8 mm offset distance, 23.5 mm radius, and 30.2 mm depth, an economic improvement of 16.9% can be achieved with a power loss of 2.4% only.