Abstract: To explore the influence of different operating parameters on the combustion process of idling, the cyclic variation of diesel engine has been studied with different control parameters on the engine test bench, and the influences of different injection timing, coolant temperature and injection pressure on the combustion cycle fluctuations have been analyzed. The results showed that when the injection timing varied from 12 ℃A BTDC to 2 ℃A BTDC, minimum rate of maximum explosion pressure fluctuation and cycle-to-cycle variation were got at 6 ℃A BTDC, which presented a high side but low middle figure. While coolant was varying from 30 ℃ to 80 ℃, the rate of maximal explosion pressure flucturation and cycle-to-cycle variation decreased from 7.64% and 1.61% to 3.47% and 0.67%, respectively, and the cycle-to-cycle variation of idling condition could be sufficiently decreased. And the increase of injection pressure from 30 MPa to 40 MPa can promote the progress of fuel atomization, evaporation and mixed. While the pressure is too high, the burn center approach to the TDC, which leads to the result that the combustion got uncontrolled. The maximum explosion pressure in cylinder and the IMEP also show a strong correlation in the stable idle process.