A high-speed germanium (Ge) waveguide photodiode (PD) is one of the key components of an integrated silicon photonics platform for large-capacity data communication applications, but the parasitic parameters limit the increase of its bandwidth. Several studies have been reported to reduce parasitic parameters, at the cost of compromising other performances. Here, we propose and investigate a bandwidth-boosting technique by comprehensively engineering the parasitic parameters. Experimentally, a bandwidth up to 80 GHz is realized for vertical positive-intrinsic-negative (PIN) Ge PDs without decreasing the responsivity and dark current, indicating that parasitic parameter engineering is a promising method to promote high-speed performance of Ge PDs.