The calculation model of rotary engine was established. Combined with the self-made program in CFD software, a 2-D dynamic simulation of the combustion process of the rotary engine was made and verified by using the experiment results. On this basis, the evolution process of combustion in direct injection LPG engine was obtained, the combustion characteristic of the engine with the three representative positions of nozzle were analyzed and the best installation position of nozzle was determined. Then a further study was conducted on the effect of the injection time and injection duration on combustion process in the rotary engine with the optimal installation position of the nozzle. The results showed that in a given speed, fuel injection direction, injection speed, injection duration and spray cone angle, the engine had a best injection advance angle that could lead to a reasonable distribution of mixture gas in the combustion chamber, a high maximum combustion pressure, maximum heat release rate and maximum pressure rise rate, a short combustion duration, and a high thermal efficiency. When the nozzle located in long axis vertex, the stability and continuity of combustion was better than the other two positions. The peak combustion pressure of the best injection advance angle was the highest. Under the optimal injection advance angle and with the other conditions unchanged, the maximum combustion pressure and maximum heat release rate appeared to reduce when changed the injection duration, but the impact was not significant. The combustion characteristics were still satisfactory, and the peak combustion pressure could reach to 6MPa above.