The research on factors affecting injection quantity fluctuation for main injection was performed. The experiment was conducted by measuring the fuel quantity and flow rate for pilot and main injection with EFS IFR8420. The factors influencing were given as follows: main injection quantity fluctuated remarkably with short dwell time and small pilot injection quantity, with increasing dwell time between pilot and main injection, main injection quantity fluctuated periodically and the fluctuation range attenuated gradually. It is concluded that the injection quantity fluctuation is caused by the fluctuation of rail pressure generated by the pilot injection. 