The combustion characteristics were studied by using a 2.0 L GDI engine at medium and low speeds part-load. The effect of intake tumble and injection timing on combustion characteristics was analyzed. The results showed that increasing intake tumble could enhance airflow and combustion velocity at the operation condition of 1000 r/min, 0.1 MPa brake mean efficient pressure (BMEP). However, the advantage of intake tumble reduced when engine speed increased. At operation condition of 3000 r/min , 0.3 MPa BMEP, increasing intake tumble could cause combustion deterioration. The effect of injection timing has greater influence on mixture formation and combustion stability at operation condition of 3000 r/min than operation condition of 2000 r/min, and there is the best injection timing value between 300°CA BTDC and 320°CA BTDC at every operation condition. 