On the basis of brief introduction of DC voltage measurement system structure and protection principle concerned of Baoji high voltage director current（HVDC） converter station， the reason of DC system protection mal-action is deeply analyzed in detail. The analysis shows that the protection action concerned is correct， but the abnormal operation of the measure device of direct current voltage divider directly causes pole blocked. In order to prevent the protection malfunction， the measuring device should be redundant and completely independent from the protection of the loop structure and hardware configuration.