The work aims to improve the high temperature oxidation resistance of AlMo0.5NbTa0.5TiZr refractory high- entropy alloys, AlMo0.5NbTa0.5TiZrBx (x=0, 0.02, 0.06) refractory high-entropy alloys were prepared by a non-consumable vacuum arc melting technology and the effects of B on the microstructure and high temperature oxidation resistance were investigated by a series of high temperature oxidation experiments, XRD, SEM-BSE and EDS. The results show that the microstructures of as-cast AlMo0.5NbTa0.5TiZr alloy has the characteristic of dendritic morphology with Al-Ti-Zr rich BCC1 phase in black interdendritic zone, Mo-Nb-Ta rich BCC2 phase in bright dendritic zone and Al-Zr rich phase between the dendritic zone and the interdendritic zone. The addition of trace amounts of B (x=0.02 and x=0.06) does not change the phase composition but refine the dendritic structure of the alloy. For the addition of B with x=0.06, the exothermic peak value decreases from 0.95 W/g to 0.05 W/g, whereas the peak temperature for oxidation reaction increases from 880 ℃ to 1020 ℃. B addition can alleviate the exfoliation of the oxidation film formed in the process of the short-term oxidation and prevent the disastrous oxidation of AlMo0.5NbTa0.5TiZr alloy during long-term oxidation. Oxidized at 800 ℃ for 50 h, the complex protective oxidation layer with Nb4Ta2O15 and AlNbO4 are formed on the surface of AlMo0.5NbTa0.5TiZrB0.06. Therefore, B addition can not only obviously reduces the oxidation weight gain rate but also greatly enhance the oxidation resistance of the alloy oxidized at both 800 ℃ for 3 h and 800 ℃ for 50 h.