In this study, we analysis an optical receivers using the optical preamplifier in a spectrum-sliced WDM systems. The average numbers of photons/bit, for an $10^{-9}$ error probability, counts using the OOK and FSK transmission. As a result, the theoretical sensitivity for PIN receiver and optical preamplifier receiver are approximately $9.2 imes10^4;and;7.2 imes10^2$ in the m=20, respectively. Also, the average numbers of photons/bit, for and given error probability, theoretical receiver sensitivity for Gaussian method and k-square method are approximately $9 imes10^2;and;2.16 imes10^2$ in the m=40, respectively. And the average numbers of photons/bit, for an given error probability, theoretical receiver sensitivity, OOK and FSK transmission are approximately $1.9 imes10^2;and;3.1 imes10^2$ in the m=20, respectively.