The invention relates to borosiloxane and a preparation method thereof. According to the method, a hydrolysis solvent and a boron compound are mixed proportionally and added to a reactor, the temperature keeps from 10 DEG C to 35 DEG C, and a mixed monomer is slowly dropwise added under the stirring condition for a hydrolysis reaction; the dropwise-adding time is controlled in 3-6 hours, the reactor is slowly heated to the temperature ranging from 55 DEG C to 65 DEG C after the dropwise-adding is finished, a stirring reaction is performed for 1 hour, and still standing is performed; acid water on the lower layer is separated and neutralized to be neutral, a saturated NaCl solution is used for washing, and an oil phase material is obtained; reduced pressure distillation is performed to remove the solvent, a catalyst I is added, the mixture is heated to 110-130 DEG C, and the rearrangement reaction is performed for 3-5 hours; the mixture is heated to 150-160 DEG C, the splitting cyclization reaction is performed for 2-5 hours, the reaction is finished, the mixture is cooled, and a polyborosiloxane prepolymer is obtained; an end-capping reagent and a catalyst II are added to a system and heated to 110-130 DEG C, and react for 2-4 hours; and pressure is reduced to remove a low-boiling-point substance, and a polyborosiloxane resin is obtained. The borosiloxane and the preparation method have the advantages of good temperature resistance, ageing resistance, impact resistance, toughness and low-temperature flexibility, high adhesion and the like.