The invention relates to a large-section water-rich saturated fine sand railway tunnel excavation method. The method includes the following steps that firstly, pilot tunnels of double side walls are constructed in the lower half section of a positive hole of a tunnel in advance and are excavated with sub-steps, temporary inverted arches are arranged on the pilot tunnels respectively, rings are formed in a sealed and segmented mode, and sub-steps are used for carrying out look-ahead dewatering and drainage on the pilot tunnels; secondly, a method that an arc pilot channel is used for reserving core earth is used for excavating the upper section of the positive hole of the tunnel, the core earth is used for stabilizing a tunnel face, arc excavation is performed in place at a time, the positive hole primary supporting steel frame is spliced in place at a time, the temporary inverted arches and a middle partition wall which are next to the tunnel face are constructed to make an upper portion supporting structure form a ring in a sealed mode, the sub-steps are manually cut to excavate the core earth, an overall positive hole primary supporting lagging jack structure is rapidly established, the inverted arches are cast and concrete filling can be carried out in time; thirdly, a result is analyzed according to monitoring measurement data, the inner side wall in the hole, the temporary inverted arches and the middle partition wall are detached in a segmented mode and a composite lining is constructed in time. By means of the method, the construction problem that the self stability of the fine sand is poor and the fine sand rapidly deteriorates when encountering water and surges outwards like quicksand is solved.