The measurement of piston noise is a critical issue in coherent combining of fiber lasers. This paper presents a strip extracting algorithm, which get the amplitude of phase noise in fiber amplifier. By this algorithm, phase noise of fiber amplifier working in 10-3 W level is measured and real-time controlled by high-speed digital processing circuit and active segment mirrors. Experimental results indicate, that after closing the loop, fringe contrast improves from 0.09 to 0.25, intensity value of piston noise decreases from 1053.4 nm down to 116.7 nm; the calibration precision and bandwidth of the system achieve 1/10λ and ～50　Hz, respectively.