Phase locking is one of the key issues for fiber laser array applications, like coherent beams combining and fiber laser phased array. These applications can be applied in fiber laser systems like laser radar, target tracking, active illumination, free-space laser communication and direct energy. Phase locking is aimed at stabilizing the wave-front phase at the pupil of each element in the fiber laser array, which is mainly caused by the path length fluctuations between uncommon lengths of fiber. Most existing fiber array systems compensate for the piston dif-ference through measuring the output phase of the array by sampling the outgoing beam using free space optical devices outside the array. To avoid the complex and dumb spatial optical devices, a new technique of phase-locking control in all fiber link based on fiber coupler has been proposed. Laser beams backscattered by the fiber tips of the different outgoing fiber laser beams interfere with each other in the fiber couplers. Meanwhile, the outgoing laser beams interfere with the partial local laser beams in the fiber couplers. These interference results provide metrics for phase-locking control algorithm named stochastic parallel gradient descent (SPGD). Laser beams are then phase-locked on their outgoing fiber tips under such system. Function of the fiber coupler is forming conventional interferometry scheme. Different from existing phase-locking methods based on active phase difference measurement, phase-locking here is achieved through optimization algorithm. The main ad-vantage of such technique is avoidance of high-speed phase modulators and complex phase demodulation. This provides a potential way to realize phase locking control with light and fiber-integrated scheme. Model of such novel phase-locking system for multi-laser-beams is built and steady-state control conditions are discussed. All fiber phase-locking is achieved for two laser beams in our experiment. The two laser beams are collimated and adjusted to overlap and interfere with each other in far field. Interference patterns in far field are collected by high speed camera to judge the control performance. Experimental results show that such technique promotes the fringe visibility of the long-exposure pattern during 10 s from 0.25 in open loop to 0.82 in closed loop, under phase disturb with an amplitude of 4 wavelengths and a frequency of 2 Hz. Fringe visibility of the short exposure pattern rises from 0.65 to 0.98 correspondingly. Experimental results prove that the phase locking method pro-posed here is effective to stabilize the wave-front phase of the fiber laser array.