Abstract: The photocathode RF gun requests accurate synchronization between laser pulse and RF phase in Thomson scattering X-ray source facility. Based on theoretical analysis on mode-locked laser phase noise, methods of phase error detection and feedback control technique are discussed. An experimental system has been established including phase jitter measurement and PLL feedback control, which changes laser optical length using a PZT. The RMS laser phase jitter has reached 3.42 ps at open loop, and 1.46 ps at close loop.