A switchable multiwavelength laser with a continuously tunable spacing is proposed and illustrated based on cascaded interactions of the second harmonic and difference-frequency generations in an aperiodically poled MgO-doped lithium niobate (MgO:APPLN) waveguide. The wavelengths and wavelength spacings of the four outputs are continuously varied and controlled by changing the two input pump wavelengths and spacings within a 3-nm flattop bandwidth of MgO:APPLN. In addition, the switchable states of the four outputs can be selected by inputting proper powers of the two pumps. The number of output wavelengths can also be increased by choosing the proper APPLN structure, the total length of MgO:APPLN, and higher powers of input pump lights.