To set a reasonable range of the length of the deceleration lane in the diverge area of the left off-ramp for urban underground interchanges，the effects of the length of the deceleration lane，as well as interactions between traffic volumes and design speeds of mainline and ramp on traffic conflict in the diverge area，were exami ned.By taking the underground interchange of the Wuhan East-South Lakes Tunnel Project as an example，the VISSIM micro traffic simulation test was carried out，and the conflict rate （RCR） and exposed time to collision （TTET） were extracted.The results show that ① when the length of the deceleration lane increases，the conflict rate decreases，and the TTET increases in the diverge area；② when the traffic volumes of the mainline or ramp increase，no significant changes were observed in the conflict rate of the diverge area，while the TTET increases to varying degrees；③ when the design speed of the mainline increases，or the design speed of the ramp reduces，both the conflict rate and the TTET of the diverge area increase；④ Analysis of variance and multiple comparisons indicate that the length of the deceleration lane plays a critical role in the variations of conflict rate and TTET，and 120~170 m is generally recommended to be a reasonable length of the deceleration lane in the diverge area of the left off-ramp for underground interchanges.The maximum length should be controlled within 195 m considering the economic value of the project.