Abstract: The fluid dynamics model was used to present the three-dimensional transient computer simulation of two-stroke gasoline engine scavenging process. In order to optimize the scavenging process, the effects of the main structural parameters on the scavenging process were discussed. By computational analysis, it was found that the crank angle of scavenging port opening, the crank angle of exhaust port opening and the scavenging port circumferential angle influenced the charging efficiency obviously. And the charging efficiency became smaller as the three parameters increased. Thus it led to power decrease. Although the escape ratio decreased rapidly as the crank angle of exhaust port opening increased, it was advantageous in the engine emissions. At last, the optimization of structural parameters was proposed based on the three-dimensional transient analysis. The crank angle of scavenging port opening was 123.1°CA, the crank angle of exhaust port opening was 112.1°CA and the scavenging port circumferential angle was 72.5°. The experiments of the improved engine indicated that the power of the improved engine increased 22.4%, and the HC emission decreased 55.1%. The study shows that it is effective to optimize the scavenging process using computer simulation.