The invention discloses an anti-seepage drainage system for a low-medium radioactive waste cavern disposal repository and a waterproof system construction method. A waterproof system comprises a waterproof lining arranged in a cavern of a mountain, wherein a concrete bottom plate is arranged at the bottom of an inner cavity of the waterproof lining; a radioactive waste bucket is arranged in the inner cavity of the waterproof lining; a gap of the inner cavity of the waterproof lining is filled with internal backfilled earth; a gap between the exterior of the waterproof lining and an inner cavity of a cavern wall is filled with external backfilled earth. The drainage system comprises an inner-side drainage ditch formed along the periphery of the bottom of an inner wall of the waterproof lining in the concrete bottom plate and an outer-side drainage ditch formed along the periphery of the bottom of the cavern wall. According to the anti-seepage drainage system and the construction method, multiple waterproof and drainage systems are designed, so that groundwater which probably seep can be blocked and rapidly drained in a running stage of the disposal repository, the drainage system can also be ensured to run normally after backfilling and closing of the disposal repository, and the durability of the drainage system can further be ensured, thereby achieving tolerance to radionuclides.