The principle of phase-preserving regeneration is revealed by a simple theoretical model, that is, in the regenerated signals the linear phase shift component is dominant over the nonlinear counterpart for phase-preserving amplitude regeneration (PPAR). A Mach-Zehnder- interferometer (MZI)-nested nonlinear optical loop mirror (NOLM) PPAR scheme is proposed and verified by theory and experiment. Our experiment shows that for QPSK regeneration the noise reduction ratio in terms of error vector magnitude (EVM) is linearly dependent on the input signal-to-noise ratio (SNR) with the slope of 0.78 and the average phase disturbation is 4.37 degree, close to the theoretical value of 3.8 degrees. The influence of the optical couplers on the PPAR performance is in detail discussed.