We demonstrate the generation of a stable high-energy Q-switched resonantly diode-pumped Er:YAG laser at 1645 nm in passive and active operation modes. For the passively Q-switched Er:YAG laser, the bi-layer graphene saturable absorber (SA), which was transferred onto the highly reflective cavity mirror by an improved method, was applied to deliver a stable pulse train with per-pulse energy of about 0.1 mJ. While in the active operation mode, the voltage-ON-type rubidium titanyl phosphate (RTP) Pockels cell as an electro-optic (EO) Q-switcher allows for the generation of a stable Q-switched pulse with per-pulse energy of up to 7.5 mJ. Our work may provide a basis for the development of a high-energy Er:YAG laser at 1645 nm to fulfill specific applications.