The invention discloses an auxiliary grouting platform for passing through a large karst cave in the expanding excavation of an existing tunnel, a grouting rack with a steel framework structure is adopted, a material channel penetrating front and back is arranged at the middle part of the steel framework structure, steel plates are respectively fixedly arranged on the top wall, the left side walland the right side wall of the steel framework structure, drilling machine ascending and descending rack temporary ramps are respectively arranged between the left side and the right side of the grouting platform and a previous expanding excavation completion section of the large karst cave, and a space formed by the upper part of the grouting rack and the top part of the large karst cave forms agrouting operation space of a drilling machine. Meanwhile, the invention further provides a construction method for passing through the large karst cave in the expanding excavation of the existing tunnel. The grouting rack with the material channel designed again is combined with the drilling machine ascending and descending rack temporary ramps distributed on the left side and the right side of the grouting rack, then the parallel operations of treatment and expanding excavation of a cavity of the large karst cave are conducted, the expanding excavation efficiency of the tunnel is obviously improved, the construction period is shortened, the auxiliary grouting platform is safe and reliable and is strong in operability, and the construction safety is guaranteed.