For real application to the all-solid-state batteries, understanding and control of the grain boundaries (GBs) are essential. However, the in-depth insight into the atomic-scale defect stabilities and transports of ions around the GBs is still far from understood. Here, the first-principles investigation on the promising garnet Li7La3Zr2O12 solid electrolyte GBs has been carried out. Our study reveals a GB-dependent behavior for the Li-ion transport correlated to the diffusion network. Especially, the Σ3(112) tilt GB model exhibits a quite high Li-ion conductivity comparable to that in bulk, and a fast intergranular diffusion contrary to the former concepts. Moreover, the preference of the electron accumulation at the Σ3(112) GB was uncovered in terms of the lower Li interstitial formation energies. This phenomenon is further enhanced by the presence of the Schottky-like defect, leading to the increase in the electronic conductivity at GBs, which plays a key role in the Li dendrite growth.