Abstract: Boron Dilution Accident（UBDA） analysis plays a significant role in the reactivity accidents during the justification of Ling’ao Mixed Core and Increased Enrichment project.As the conclusion of the accident analysis will define the standard control rod position at both cold and hot shutdown conditions,of which the control rod position of hot shutdown serves as the premise of Rod Withdrawal from Subcriticality（RWS） accident analysis,the rod position of standard hot shutdown can be adjusted through UBDA analysis in an effort to provide the margin for RWS accident analysis.A detailed analysis method and conclusion of BDA at Ling’ao Nuclear Power Station are described in this paper.