The influences of B element on the microstructure and property of AlCoCrFeNiBx high entropy alloys (x denotes the atomic fraction of B element 0, 0.1, 0.25, 0.5, 0.75 and 1.0) were investigated. The AlCoCrFeNi high entropy alloy exhibit equiaxed grain morphology, then turns to dendrite structure when B content, x=0.1. The spinodal decomposition microstructure can be clearly observed in equiaxed grains. When x>0.1, both of the dendrite and the spinodal decomposition microstructure gradually disappeared, but much borides formed instead. The transformation is attributed to the high negative mixing enthalpy of Cr-B and Co-B. The microstructures of AlCoCrFeNiBx high entropy alloys changed from B2 BCC structures to B2 BCC FCC structures, finally formed B2 BCC FCC and borides mixing structures with the increased B elements. And the hardness declined from HV486.0 to HV460.7, then rose to HV615.7 as the addition of B element. The lowest hardness value is obtained when x=0.1. The compressive fracture strength shows a distinct decrease with B addition. The maximum compression strength is 2227MPa when x=0.25. But when x reached 0.75, the samples fractured during the elastic deformation due to the formation of hard and brittle borides. The coercive forces and the specific saturation magnetizations of the alloys decrease as the contents B element increase. The decreasing coercive forces show a better soft magnetic behavior.