We propose and design a germanium (Ge) waveguide laser under external phonon injection to reduce laser threshold. To take the phonon injection and the acousto-optic overlap into consideration, the theory of the photon-phonon laser action is further developed. The phononic crystal waveguide is introduced in the laser structure to intensify acousto-optic interaction, and characteristics of photon-phonon laser action in Ge waveguide are analyzed. With the external phonon injection, the two-quantum transition can be facilitated and the photon-phonon laser action is able to be established. The impacts of phononic crystal waveguide parameters, overlap of optical and acoustic fields, and phonon injection on the laser behavior are discussed. Optimal waveguide structural parameters are obtained to enhance acousto-optic interaction through the enlargement of the overlap of optical and acoustic fields. The results indicate that, for a Ge waveguide with the length of 200 μm, the threshold current is reduced to 0.2 μA and the slope efficiency reaches 0.7 W/A when the average phonon injection concentration is about 2.5 × 1021 cm-3. Our proposed scheme offers an effective approach to achieve laser oscillation in integrated Ge waveguide.