The Li+ concentration and Li+ transport channel size of garnet-type solid electrolyte Li7La3Zr2O12 are the two key factors associated with the improvement of Li ionic conductivity. Through precise control of the Li+ concentration and the Li+ transport channel size by Ta-doping (Li7-XLa3Zr2-XTaXO12, X = 0.2–1.0), synergistic regulation of the two factors is achieved. Results show that the optimum Li+ concentration seems to be at about 6.31 where the Li+ occupancy in LiO6 is high and the Li+ arrangement in LiO4 is disordering. With the Li+ concentration higher than 6.31, the occupancy in LiO4 sites remains constant but the Li+ arrangement in LiO4 becomes more and more ordering. At higher Li+ concentration of 6.52, Li+ arrangement becomes certain of ordering which should have reduced the grain conductivity, however, the grain conductivity is still as high as 9.84 × 10−4 S cm−1. This indicates that LLZO with this Li+ concentration have favorable Li+ transport channel size. The optimum Li+ transport channel size is about 12.95 ± 0.01 Å. At Li+ concentration of 6.31, the proper Li+ concentration is the main factor for the high grain ionic conductivity of 10.1 × 10−4 S cm−1. While at Li+ concentration of 6.52, the suitable Li+ transport channel size is the main factor for the high grain ionic conductivity. In conclusion, the high grain ionic conductivity at Li+ concentration of 6.31–6.52 is due to the synergistic regulation of Li+ concentration and Li+ transport channel size.