We have demonstrated phase noise measurements of ytterbium doped fiber amplifier on transient pumping process based on active phase locking using optical heterodyne detection from all-fiber Mach-Zehnder interferometer. The intrinsic and extrinsic factors during amplifying process worsen coherence and lead to randomly fluctuated phase error. Root mean square, power spectral density, and phase drift velocity were employed as quantitative values for evaluating phase error. The results implied that main frequency bandwidth of phase noise was characterized by type of pumping process, output power, and fiber amplifier components. This platform can be utilized to evaluate phase noise error directly under the different amplifier configuration and condition, and it helps find better condition of amplifier for assuring rigid beam combining performance henceforth.