Realizing a hydrogen economy strongly requires alkaline water electrolysis to achieve large-scale generation of H2, but lacks stable and efficient catalysts. The insufficient active sites in Ni(OH)2 impair the catalytic performance of alkaline HER. Herein, trivalent Al3+ is introduced to tune the in-plane anistropical strain of Ni(OH)2, optimizing the electronic structure of the basal plane to provide more active sites for enhanced alkaline HER performance. DFT calculation reveals that the in-plane strain through Al3+ facilitates the adsorption energy of H*, reduces the HER energy barrier, and adjusts the Volmer process. As a result, the optimized catalyst exhibits efficient HER catalytic performance at an overpotential of 190 mV to drive 100 mA cm-2 along with the low Tafel slope of 48 mV dec-1 in alkaline solution. The anisotropic in-plane strain engineering strategy highlights the importance of atomic engineering on the HER catalysts.