Industrial production of NH3 from N2 and H2 significantly relies on Haber-Bosch process, which suffers from high energy consume and CO2 emission. As a sustainable and environmentally-benign alternative process, electrochemical artificial N2 fixation at ambient conditions, however, is highly required efficient electrocatalysts. In this study, we demonstrate that hexagonal boron nitride nanosheet (h-BNNS) is able to electrochemically catalyze N2 to NH3. In acidic solution, h-BNNS catalyst attains a high NH3 formation rate of 22.4 μg·h-1·mg-1cat. and a high Faradic efficiency of 4.7% at -0.75 V vs. reversible hydrogen electrode, with excellent stability and durability. Density functional theory calculations reveal that unsaturated boron at the edge site can activate inert N2 molecule and significantly reduce the energy barrier for NH3 formation.