In an AC-DC hybrid grid， load rejection resulting from AC circuit breaker tripping or DC converter locking frequently leads to serious frequency overvoltage in the AC bus of a converter station. To protect the related equipment against damage， overvoltage protection is equipped both in DC and AC power systems. In practice， however， the current overvoltage protection of both systems often lacks coordination with each other， which may lead to incorrect operation. Based on an analysis of maloperation in overvoltage protection， it is pointed out that overvoltage protection on both the AC and DC system sides should meet the selectivity requirement on value setting， and the principle of setting cooperation is proposed. This method has successfully been applied to the high voltage direct current（HVDC） transmission project in Northwest China and its adjacent AC grid， greatly improving the performance of overvoltage protection in the AC-DC hybrid grid.