The wavelength conversion of picosecond optical pulses based on the cascaded second-harmonic generation–difference-frequency generation process in a MgO-doped periodically poled lithium niobate waveguide is studied both experimentally and theoretically. In the experiments, the picosecond pulses are generated from a 40\text{\hspace{0.17em} GHz} mode-locked fiber laser and two tunable filters, with which the lasing wavelength can be tuned from 1530 to \text{1570 \hspace{0.17em} nm} , and the pulse width can be tuned from \text{2 \hspace{0.17em} to \hspace{0.17em} 7 \hspace{0.17em} ps} . New-frequency pulses, i.e., converted pulses, are generated when the picosecond pulse train and a cw wave interact in the waveguide. The conversion characteristics are systematically investigated when the pulsed and cw waves are alternatively taken as the pump at the quasi-phase-matching wavelength of the device. In particular, the conversion dependences on input pulse width, average power, and pump wavelength are examined quantitatively. Based on the temporal and spectral characteristics of wavelength conversion, a comprehensive analysis on conversion efficiency is presented. The simulation results are in good agreement with the measured data.