The invention discloses a soft soil layer rotary excavating pile construction method. During construction, a surface soft soil layer is replaced with a sand and stone cushion layer so that the sand and stone cushion layer can bear construction self-weight load of a rotary excavating pile machine, then an all-steel retaining cylinder is embedded, and the problems of necking down and pile hole shifting in a construction process of cast-in-place piles of the soft soil layer are solved. According to the soft soil layer rotary excavating pile construction method, by replacing the soft soil layer with the sand and stone cushion layer and by means of the all-steel retaining cylinder process, efficient hole-forming of pile rotary excavating of the soft soil layer is achieved, the process is simple, and quality safety is guaranteed.