The invention discloses a capacity reduction treatment system and method for medium-low radioactive solid waste. The system comprises a crusher, a sorting box, a plasma gasifier, a plasma fusion furnace, a burning chamber, a quench tower, a ceramic filter, an activated carbon adsorption tower, a deacidification washing tower, an efficient air filter, a chimney, a fused vitreous body receiver and aconveyor. Firstly, the medium-low radioactive solid waste is crushed by using a crushing device and then conveyed to the sorting device for sorting, the combustible waste is conveyed into the gasifier, organic components are subjected to a gasification reaction, and generated ash and noncombustible waste are collected and conveyed into the fusion furnace for vitreous fusion treatment, so that radioactive nuclide is fixed into the fused vitreous body; the vitreous body is cooled, barreled and stored by an automatic receiving system, and generated smoke is purified and then reaches the comprehensive emission standard of atmospheric pollutants, so that up-to-standard emission is achieved. The capacity reduction treatment system has a good capacity reduction effect and is high in safety reliability and nuclide stability.