针对特大跨度四线铁路隧道，其开挖面积相当于6倍普通单线铁路隧道，开挖成洞难度大，尚无成功先例。以六（盘水）沾（益）铁路乌蒙山2号出口四线车站隧道工程为背景，通过数值模拟和现场监测，开展特大跨度隧道施工工法研究。研究结果表明： 1）通过“撑索转换”，即利用“外锚”替换“内撑”，实现拱部临时支撑的安全拆除，降低拱部支护体系转换过程中的施工风险，并且突破传统软岩隧道施工中拆撑步长、跳拆等限制，提高了拆撑安全性及灵活性，同时为后续施工提供了较大的工作面。2）通过“以索代撑”，即利用锚索取代临时横撑，避免临时撗撑拆除风险，使得开挖更加灵活。3）采用三台阶“撑索转换”及“以索代撑”施工工法，实现了特大跨度隧道台阶法施工。

The construction of ultra largespan fourtrack tunnel, of which the excavation area equals to 6 times of that of traditional singletrack railway tunnel, is difficult; and there is no successful precedent. The construction methods for ultra largespan fourtrack deep tunnel on Wumengshan No. 2 exit section on LiupanshuiZhanyi Railway are studied by numerical simulation and site monitoring methods. The results show that: 1) By using temporary support replaced by prestressed anchor, the temporary support of arch is successfully dismantled, the replacing construction risk of arch is reduced; the disadvantages of traditional soft rock tunnel construction are overcome; the safety and flexibility of support dismantling are improved; and large working face can be provided for followed construction step. 2) By using temporary crossbrace replaced by prestressed anchor cable, the temporary crossbrace dismantling risk is avoided; and the construction flexibility is improved. 3) The threebench excavation of ultra largespan tunnel is realized by using temporary support replaced by prestressed anchor and temporary crossbrace replaced by prestressed anchor cable.