Abstract: It is one of the main ideas of developing ATF cladding materials to improve the accident resistance of zirconium alloy by coating. In the existing research on the protection mechanism of the coating on zirconium alloy substrate, the main focus is on the oxidation resistance of the coating. It is often concluded that all kinds of coating can effectively improve the accident resistance of zirconium alloy. But in fact, the embrittlement of zirconium alloy cladding under accident conditions depends not only on the degree of oxidation, but also on the change of hydrogen content in zirconium alloy. The increase of hydrogen content will greatly reduce the plasticity of zirconium alloy under accident conditions. In this study, the high-temperature steam oxidation quenching experiment at 1 200 ℃ was carried out on the AlCrNbSiTi high entropy alloy coating zirconium alloy prepared by multi-arc ion plating process. The plasticity of zirconium alloy coated with high entropy alloy after oxidation was evaluated by ring compression. The embrittlement mechanism of zirconium alloy was investigated by weighing, metallography, scanning electron microscope, electron probe X-ray microanalysis and inert gas pulse melting infrared absorption method. After oxidation for 335 seconds, no oxide layer is observed on the coating side, which indicates that the coating can effectively prevent the oxidation of zirconium matrix under this oxidation condition. The result of testing the hydrogen content of the matrix is about 100 ppm (under the same conditions, the Zirlo alloy is about 10 ppm), and the result of testing the offset strain of the matrix is more than 20%, which indicates that the coating can not prevent the hydrogen content in the zirconium alloy matrix from increasing under this oxidation condition, but plays a reverse role. However, 100 ppm hydrogen is not enough to reduce the plasticity of high matrix, and oxidation still plays a major role. After oxidation for 500 seconds, no oxide layer is observed on the coating side, which indicates that the coating can still effectively prevent the oxidation of zirconium matrix under this oxidation condition. The result of testing the hydrogen content of the matrix is about 240 ppm (under the same conditions, the Zirlo alloy is about 10 ppm), and the result of testing the offset strain of the matrix is more than 5%. This indicates that the increase of hydrogen content caused by the coating is the main reason for the loss of plasticity of the zirconium alloy matrix. The results show that high entropy alloy coating can effectively prevent the oxidation of zirconium matrix, and the loss of plasticity of zirconium matrix may be due to the synergistic effect of hydrogen embrittlement and oxidation embrittlement.