Different modulation formats may be required in different optical communication networks. Format conversion from intensity format to phase format is necessary to ensure the functions between long-haul transmission networks and metropolitan area networks. Cross-phase modulation (XPM) in silicon waveguides provides a promising way to realize all-optical integrated format conversion since a nonlinear phase shift is induced to the probe by the incident signal power. An on-off keying (OOK) signal can be converted to differential phase-shift keying (DPSK) signal if nonlinear phase shift of π is achieved. We numerically investigate the nonlinear phase shift caused by XPM in silicon waveguides by considering the influences of the walk-off effect, group-velocity dispersion, and nonlinear losses including two-photon absorption (TPA) and free-carrier absorption (FCA). The nonlinear phase shift is tried to be enhanced through waveguide design and wavelength management. The walk-off effect can be minimized by carefully choosing the zero dispersion wavelength of the used silicon waveguide and setting the signal and probe wavelengths symmetrically. Low and flat dispersion is beneficial to acquiring a large nonlinear phase shift. TPA and FCA will greatly reduce the nonlinear phase shift obtained from XPM and they should be effectively suppressed in order to realize high-quality format conversion.