The invention relates to a compound boron rubidium sulfate, a boron rubidium sulfate crystal and a preparation method thereof. The chemical formula of the compound is Rb5BS4O16, the molecular weight is 822.40, the chemical formula of the compound boron rubidium sulfate crystal is Rb5BS4O16, the molecular weight is 822.40, the compound boron rubidium sulfate crystal belongs to a tetragonal system, the space group is P43212, the cell parameters are as follows: a is equal to 10.148(4) A0, c is equal to 16.689(14) A0, Z is equal to 4, V is equal to 1718.8(17) A3, the boron rubidium sulfate compound is prepared by a solid reaction process, the compound is used for growing crystal through a flux growth method to obtain the boron rubidium sulfate crystal, the prepared crystal has a large dimension of at least centimeter level, and the crystal has the advantages of wide transmission waveband, fast preparation speed, simplicity in operation, short growth period, good mechanical property and easiness for processing, and the like.