The invention relates to a nonlinear optical crystal of a potassium borate chloride compound and a preparation method as well as application thereof. The chemical formula of the compound is K3B6O10Cl, and the molecular weight of the compound is 377.61. The crystal does not have a symmetric center and belongs to a trigonal system and a space group R3m, and cell parameters are as follows: a=10.0624(14), b=10.0624(14), c=8.8361(18), Z=3, and V=774.8 (2). The powder frequency-doubled effect reaches about 3 times of KDP(KH2PO4), the Mohs hardness is 4-5, and the light transmission waveband is 165nm-3460nm. A solid phase reaction method is used for synthesizing the compound, and a cosolvent method is used for growing the crystal. The crystal has the advantages of simple operation, low cost, large size, short growth cycle, few inclusion enclaves and larger mechanical hardness, and can be cut, polished, processed and saved easily. The crystal is used for generating twofold frequency or three-fold frequency or fourfold frequency or fivefold frequency harmonic light output for laser beams of which the wavelength is 1064nm.