Using the first-principles method, we have studied the structure transition of two-dimensional hexagonal boron nitride (2D h-BN) under large uniaxial strain. The strain is applied by changing the values of Lx and Ly, which correspond to the lengths of primitive cell in the directions perpendicular and parallel to B—N bonds, respectively. For the large asymmetrical tensile strain perpendicular to B—N bonds, the rhombic structure is stable when Lx≤0.3388 nm. As the strain increases, the system transforms from the rhombic structure to a rectangular structure consisting of interlaced interacting BN chains, which becomes stable when Lx≥0.3488 nm. When the strain further increases, the system finally changes into the one comprised of isolated BN chains. For the 2D h-BN with large asymmetrical tensile strain distribution parallel to B—N bonds, there is no stable rectangular structure and the system becomes the one composed of isolated BN chains when Ly>0.571 nm.