A multichannel polarization-entangled photon-pair source in an MgO-doped periodically poled lithium niobate (MgO:PPLN) waveguide is proposed. Based on type I quasi-phase-matched spontaneous parametric down conversion in a single MgO: PPLN waveguide placed inside a Sagnac interferometer and pumped by monochromatic light, a source capable of supporting tens to hundreds of channels of polarization-entangled photon pairs in fiber communication bands simultaneously can be achieved. An inherent channel switch of this source is investigated, which will be significant for future entanglement distribution networks.