According to the collapse, borehole diameter shrinkage, slip of drill bit, large thickness of sediment in the bottom appeared in a viaduct project in soft soil layer with rotating drilling rig, the accident causes were analyzed; and casing lengthening, bit structure improving, drilling process optimizing, borehole cleaning by air-lift reverse circulation and slurry purifying methods, etc. were adopted with good effects. The construction experiences could be reference to the similar projects.