In RF photoinjector, the diode-pumped self-mode-locked laser(GE-100) is used as the laser seeds in the driving laser, and the laser pulse's timing jitter is reduced down below 2ps by means of phase-locking technology. The detail of the timing jitter is discussed, the laser pulse's timing jitter is determined by the phase-noise of the driving signal and the instability of the mode-locked laser. So the high stable mode-locked laser and the driving signal of low phase-noise is required to the low timing jitter.