As both the external input jitter and the internal phase noise jitter impact on the bandwidth of Phase-Locked Loop(PLL) in an Optical Transponder Unit(OTU), the bandwidth of the PLL was designed optimally to solve the contradiction of the wavelength selection. With consideration of the external input jitter and the internal phase noise, the bandwidth algorithm of the PLL was comprehensively analyzed, and an optimization algorithm of the bandwidth for PLL in the OTU was deduced through improving calculation and analytical methods from formers. The optimal bandwidth of the PLL can ensure the lower jitter noise of clock output in the OUT. The optimization algorithm was applied in the OTUs with 2.5 Gbit/s and 10 Gbit/s ,respectively to analyze the loop parameters and the optimal parameters of the PLL and the output jitter value of a regenerator were acquired quantitatively and qualitatively. The experiment shows that the optimization algorithm is superior to traditional algorithms and more suitable for OTUs.