The invention discloses a large-scale karst cavity treatment method for a karst tunnel. In the construction tunnel, the section traversing the interior of a large-scale karst cavity is a traversing karst cavity section, the left and right sides of which are suspended; and the size of the large-scale karst cavity is gradually increased from top to bottom, and the karst cavity is divided into an upper karst cavity and a lower karst cavity by the traversing karst cavity section. The treatment method comprises the following steps of: 1, digging a side slope, and performing reinforcing construction, namely digging the inner wall of the upper karst cavity to form the side slope; 2, backfilling rock slag, namely filling the lower karst cavity by adopting the rock slag to form a filling base layer; 3, grouting and reinforcing the filling base layer, namely reinforcing the filling base layer by using a stabilizing pile and obtaining a road cutting I; and 4, performing paving construction of the traversing karst cavity section on the road cutting I. The method is reasonable in design, simple and convenient in construction, safe and reliable in the construction process, short in construction period and good in treatment effect, and can solve the problems of high construction safety risk, high treatment and construction difficulty, long treatment time, poor treatment effect and the like in the conventional large-scale karst cavity treatment.