A single crystal K2CaB8O26H24 with sizes of 11 mm × 6 mm × 5 mm was grown by room temperature solution method. It belongs to the orthorhombic system, space P212121, and lattice parameters a=1.15520(2) nm, b=1.24751(3) nm, c=1.65768(4) nm. The measurement of the infrared spectrum indicates that the basic anionic group is the BO33- and BO45-. The transmittance spectrum shows that the UV cut-off wavelength of K2CaB8O26H24 is about 200 nm. Its powder second harmonic effect is about 0.3 times of KDP crystal.