In order to improve the bipolar neutral bus differential protection reliability, the improvement measure of protection is proposed according to the existed deficiency. The bipolar neutral bus differential protection performance exists deficiency, which has led to monopole or bipolar forced outage event by one component fault. The performance of bipolar neutral bus differential protection is analyzed combining with actual forced outage events, which includes: 1) the necessity and way of non-electricity signals adopted by protection; 2) the protection logic of blocking by control pole criterion and operating strategy of closing NBGS in the case of bipolar operation. According to the analysis results, the improvement measure of protection is proposed. The measure can be not only improve protection reliability, but also increase the efficiency of HVDC maintenance work. At present, the measure is implemented in multiple HVDC system, and then the bipolar neutral bus differential protection of these HVDC system operates well after adopting the measure.