A low noise laser system for atom interferometry is realized with phase-locked fiber lasers, where the performance of the OPLL is greatly enhanced by the FEOM feedback loop and the narrow linewidths. The laser system demonstrated contribute 2.2 mrad per shot to the interferometer noise and permit continuous long-term operation for more than 115 hours without relocking in the field test. Also, the mobile gravimeter equipped with this phase-locked laser system reaches a sensitivity as good as 29 uGal/\sqrt{Hz} and a resolution of 1.1 uGal within 1500 s, demonstrating performances comparable to the state of the art.