Linear gain and gain slope are proposed to describe the amplification and reshaping characteristics of multilevel all-optical amplitude regenerators. The reshaping range, the optimal operating point, and the number of regenerative levels can be determined based on the gain slope curve of the multilevel all-optical amplitude regenerators. We focus on Mach-Zehnder interferometer (MZI)-based multilevel all-optical amplitude regenerator with a gain slope curve of cosine oscillation. The smaller the signal oscillating amplitude is, the more the regenerative level numbers are. The results show that the amplitude noise of pulse-amplitude-modulating signal which is through MZI multilevel all-optical amplitude regenerator can be suppressed greatly, while the phase jitter is increasing.